398 



GEOLOGICAL SUCCESSION OF ATSTIMALS. 



with unequal valves, having peculiar appendages in the interior. 

 The Lepttena alternata, found very abundantly in the Trenton 



limestone, is 



377. one of those 



shells. The 

 only fossils yet 

 found in the 

 Potsdam sand- 

 stone, the old- 

 est of all fossi- 

 liferous depo- 

 sits, belong al- 

 so to this fa- 

 mily (Lingula 

 prima) . Be- 

 sides this, there 

 are also found 

 some bivalves 

 of a less un- 

 common shape 

 (Avicula de- 

 cussata) ; [and 

 in the upper 

 stages of the 

 Silurian group 

 in England we 

 find Orthis or- 

 Orthis navicularis, 



bicularis (1), Terebratula navicula (2), 



(3) Pentameus Knightii (4), Atrypa affinis (5), fig. 377.] 



663. The gasteropoda are less abundant ; some of them 

 are of a peculiar shape and structure, as Bucania expansa, 

 Euomphalus hemisphcericus. Those more similar to our 

 common marine snails have all an entire aperture ; those with 

 a canal being of a more recent epoch. 



664. Of the cephalopoda we find some genera not less 

 curious, part of which disappear in the succeeding epochs ; 

 such, in particular, as those of the straight, chambered shells 

 called orthoceratites, some of which are twelve feet in length 

 (Orthoceras ventricosum). There are also found some of a 

 coiled shape, like the ammonites of the secondary age, but 

 having less complicated partitions (Lituitesgiganteus, 7). The 

 true cuttle-fishes, which are the highest of the class, are not 



