Ceratopsia
Ceratopsia thrived in NorthAmerica andAsia during theCretaceous Period (about 146 to 65.5 million years ago). Their name means "horned face" and indeed many of them did feature bony horns projecting from their skulls. ■
A、The horns may look quite predatory, butCeratopsians were herbivores¹, so their horns were defensive in nature and may have been used to enforce order in the group. ■
B、Not allCeratopsians had horns, but all had beaklike mouths. ■
C、The fossil record has revealed three distinct families amongCeratopsia: Psittacosaurids, Protoceratopsids, andCeratopsids. Triceratops, a member of theCeratopsid family, is probably the best known species ofCeratopsiA、With its facial horns and neck display, it exhibits many of the features commonly associated with this dinosaur group. ■
D、However, there are species ofCeratopsia that are quite different from the familiar Triceratops.
The Psittacosaurids comprise the Psittacosaurus genus² and the Hongshanosaurus genus.Although not much fossil data are available for Hongshanosaurus, comparisons with Psittacosaurus can be made.Evidence indicates that both were roughly deer-sized bipeds dwelling inEastAsiA、Hongshanosaurus had beak points on both upper and lower jaws, while only the upper jaw of Psittacosaurus was pointeD、Much more fossil evidence is available for Psittacosaurus species.At least one species had long quills along its back and tail. Researchers speculate these quills were used for display during mating or fighting.As earlyCeratopsians, the Psittacosaurids had many anatomical features that would show up in similar or interestingly modified forms in later species, but none of the Psittacosaurids had the neck frills or facial horns that came to be associated with theCeratopsians.
Although their name suggests that the Protoceratopsids were the first of this dinosaur group to have horned faces, they, in fact, lacked the well-developed horns of later species. While they more closely resembled theCeratopsids on the whole, they were still relatively small and hornless. Protoceratops, a six-foot-long quadrupedal herbivore, is a representative ProtoceratopsiD、Fossil examples found inChina and Mongolia show that it had a turtle-like beak and a neck frill, a version of which would show up among the more familiarCeratopsids. While Protoceratops did have some knobby points on its skull, the points only slightly hinted at the impressive horns that appeared on theCeratopsids.
With the arrival of Triceratops and other advancedCeratopsids,Ceratopsia reached new levels of sophistication, variety, and size.AllCeratopsid fossils have been found in western NorthAmerica and reveal that they were quadrupedal herbivores with beaked jaw tips and rows of teeth specialized for shearing. The family featured a remarkable array of horns, neck frills, and spines. These neck frills, manifesting in various shapes and sizes, probably were too thin to be practical as a defense against predator species, and researchers speculate that they were used during mating displays, for anchoring jaw muscles, for regulating body temperature, or for a combination of these purposes. MostCeratopsids had two long brow horns and a short horn on the nasal ridge.Because theCeratopsids were vegetarian and fairly large (up to 30 feet long and 10 feet tall), researchers think that the horns, along with the neck frills, largely served functions of competition, whether in displays or combat. Moreover, since intraspecific conflicts often are tests of strength rather than fights to the death, thin neck frills may have been sturdy enough to provide some protection during such confrontations.
There is evidence ofCeratopsian species that do not neatly fall into the familial categories as describeD、While these species appear closely related toCeratopsia, they tend to be

San Francisco: ItsContemporary Issues
Famous for its beautiful setting, San Francisco is built on a series of steep hills located on the northern tip of a peninsula at the entrance to San FranciscoBay. The bay and its extensions, which include San PabloBay and SuisunBay, constitute one of the great natural harbors of the world, embracing nearly 1,200 sq km (more than 450 sq mi) of water.Because of this, San Francisco was once the major PacificCoast seaport of the United States. Today the city is an important center for finance, technology, tourism, and culture. The city was named after San FranciscoBay, which in turn was named for Saint Francis ofAssisi by early Spanish explorers.
The people of San Francisco can take pride in their city’s accomplishments.
San Franciscans, and in some cases their counterparts in theBayArea, have successfully undertaken mammoth construction projects such as the San Francisco-OaklandBayBridge, the Golden GateBridge, and theBayArea Rapid Transit system.
Since at least the 1950s, San Franciscans have also earned a reputation for tolerance of and respect for diversity.

During the late 1990s the greatest problem in San Francisco’s infrastructure was the Municipal Railway. Proportionately more San Franciscans rely on public transportation than do the people in any otherCalifornia city, but riders complained of serious delays and overcrowding. Some improvements were underway by 1999, and in that year city voters also approved major changes in the organizational structure of the city’s transportation departments.
The most serious social problems facing the city are not unique to San Francisco, but some have taken on greater dimensions in the city than they have elsewhere. One such problem is homelessness.During the administration of MayorArtAgnos from 1988 to 1992, the plaza in front of city hall became an encampment for homeless people, rendering other use impossible and raising public health concerns.Agnos’s political opponents dubbed it "CampAgnos" and the situation contributed toAgnos’s defeat in 1991. The problem of homelessness persists despite the efforts of city agencies and private charities to provide shelter, health care, and drug, alcohol, and mental health treatment. In the mid and late 1990s mayors Frank Jordan and WillieBrown both sought to discourage homeless people from living in public space in the downtown area and, inBrown’s case, in Golden Gate Park. However, residents of other areas complained that because of these projects, the displaced homeless had moved into their neighborhoods.
In other areas the city has made some progress toward addressing social problems.As was true across much of the nation, the crime rate in San Francisco dropped in the 1990s, as did the rate of drug-related violence. In addition, some public homing projects in San Francisco that were especially prone to violence and drug-related activity were razed and rebuilt with designs considered less likely to encourage those activities. Other public homing projects received stepped up security patrols.
Some social critics have pointed to an increasing economic and social polarization of San Francisco’s population. Those who work in finance or high-tech fields are increasingly affluent, pushing rents and home prices to among the highest levels in the nation.At the same time, people who labor in the service sector often work for the minimum wage, cannot share the affluent lifestyles around them, and are hard-pressed to afford rising rents. The disappearance of many unionized jobs in manufacturing and

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