Handbook of Paleontology 261 



and Plectambonites. Plectambonites sericeus is very com- 

 mon both in the middle and upper series of rocks, even 

 to such an extent that thick layers of limestone are almost 

 entirely composed of it. The Platystrophia group is prac- 

 tically confined to the Ordovician. Among gastropods 

 forms with spirally coiled shells have increased in num- 

 ber but both high and low-spired forms abound. Among 

 the important genera are Eotomaria, Hormotoma, Tro- 

 chonema, Raphistoma, Bellerophon types, and Maclurea. 

 The last named, a flat-spired, left-handedly coiled form, 

 is particularly characteristic of the period, though it al- 

 ready occurs in the late stages of the Canadian. Although 

 not uncommon, neither the gastropods nor the bivalve 

 mollusks (pelecypods or lamellibranchs) approach any- 

 where near the relative importance they have in modern 

 times. Pelecypods are still comparatively rare as fossils, 

 and are more abundant in the sandstones and shales of 

 the period than in the limestones. Characteristic genera 

 are found in the peculiar Ambonychia and Byssonychia 

 which possesses a large wing on one side of the beak. 

 Mussellike forms, such as Modiolopsis, are also found, 

 but similar forms occur also in younger (later) beds. 

 The cephalopods were probably the most powerful ani- 

 mals of the Ordovician and showed great diversity. They 

 all had shells of the type of the Pearly Nautilus of the 

 present day and these shells were straight (Orthoceras), 

 curved (Cycloceras) or tightly coiled (Trocholites) . The 

 straight-shelled forms were most characteristic and 

 reached their culmination in this period. Other genera 

 are Oncoceras, the straight form with rapidly tapering 

 cone, Gonioceras, very characteristic of the Middle Ordo- 

 vician, and the coiled Enrystomites also represented in 

 the Upper Canadian. Ordovician trilobites fall far short 



