354 



GEOLOGY. 



The Ordovician gastropods. — The gastropods were very fairly 

 represented in the early Ordovician fauna. There were present not 

 only the simpler shell-forms, which consist merely of a low cone with a 

 vertical or slightly curved axis, like the capulids (Fig. 161 i, /), but 

 also the three leading types of the spiral shell, the depressed concave 

 coil (Fig. 161, k, I), the low spire (Fig. 161, n) and the elevated spire (Fig. 

 161, g and a). The last made a relative gain as the period progressed. 

 Perhaps no form of early Paleozoic life so closely and so obviously 

 resembled the modern forms as the gastropods, which even the unin- 

 structed recognize as ." snails. " The Conularia (Fig. 161, r) represents 

 the suborder pteropods. 



The Ordovician pelecypods. — The pelecypods (lamellibranchs), like 

 their representatives (the clams) of to-day, seem to have been fond of 

 muddy or sandy bottoms, for they are rather rare in the limestone 



1 m a 



Fig. 162. — Ordovician Pelecypods (Lamellibranchs). a, Pterinea demissa (Conrad), 

 exterior of left valve; b, Lyrodesma cincinnatiensis Hall, interior of right valve, 

 showing a primitive type of hinge; c, Byssonychia radiata (Hall), exterior of left 

 valve; d, Vanuxemia dixonensis M. and W., interior of right valve, showing the 

 hinge and muscular impressions; e, Ischyrodonta decipiens Ulrich, interior of 

 right valve, showing the hinge and muscular impressions; f-g, Ctenodonta nasuta 

 (Hall), exterior of right valve and a view of the hinge; h, Modiolopsis arguta (Ulrich), 

 exterior of the left valve, and a view of the hinge; i-j Ctenodonta recurva Ulrich, 

 exterior of right valve and a view of the hinge; k, Rhytimya radiata Ulrich, 

 exterior of the right valve, I, Orthodesma rectum H. and W., exterior of the right 

 valve; m, Ctenodonta pectunculoides Hall, interior of the left valve, showing the 

 hinge; n, Technophorus divaricatus Ulrich, exterior of the left valve. This is 

 usually described as a pelecypod, but it is probably a peculiar bivalve crustacean. 



beds of the earlier and middle Ordovician, but increase in abundance 

 as the deposits grade into the shales of the later Ordovician. On the 

 whole they were very subordinate to the gastropods and cephalopods 

 in numbers and in range, except locally. This is perhaps the more 



