2/0 MOLLUSCA. 



mostly belong to the genera Hyalea, Cuvieria, Cleodora, T/ieca, 

 Pterotheca, Tentaculites, and Conularia; but other less import- 

 ant types are known to have existed in past time. Of the 

 above-mentioned generic forms, the first three are well repre- 

 sented at the present day by living forms. The remaining 

 four are almost exclusively Palaeozoic, Conularia alone surviv- 

 ing into the earlier portion of the Mesozoic period. Not only 

 is this the case, but the forms in question all commence their 

 existence in the Lower Silurian or Upper Cambrian, and none 

 of them except Conularia transgresses the upper limit of the 

 Devonian Rocks. Lastly, almost all these forms are of com- 

 paratively gigantic size, and they differ in many respects from 

 living forms. 



In the genus Hyalea (fig. 242) the shell is globular, trans- 

 lucent, the dorsal plate ex- 

 ejfflk tended into a hood ; the aper- 

 ^/ \ Rtf/B ture is contracted, with a late- 

 X^5^ \*& ral slit on each si(1 e- The 

 fossil species are only known 



Fig. ^.-HyahaOrbignyana. Miocene J n t he Miocene and PllOCCne 



Tertiary, and the genus at- 

 tains its maximum in existing seas. Cleodora has a pyramidal 

 shell, and dates from the Miocene ; and Cuvieria (fig. 241) 

 has a cylindrical shell, and dates from the Pliocene. Both 

 these genera attain their maximum at the present day. 



The genus Theca (the Hyolithcs of Continental and American 

 palaeontologists) comprises a number of singular forms in 

 which the shell is straight, sheath- 

 shaped, tapering to a point, triangular, 

 and destitute of lateral appendages 

 (fig. 243). The mouth of the shell 

 is trigonal, sometimes closed by an 

 operculum, sometimes furnished with 

 curved lateral appendages. The length 

 of the shell is commonly about an inch 

 or an inch and a half. Nearly ninety 

 species of this genus are enumerated 

 b 7 M - Barrande, distributed in the 

 and us opercuium. (After Sal- Upper Cambrian, Silurian, and Devon- 



ter.) From the Tremadoc Slates r . , , 



(Upper Cambrian ?> ian formations, but not extending into 



the Carboniferous period. 



The genus Pterotheca of Salter is exclusively Silurian, and is 

 characterised by having the " shell transversely oval, bilobed, 

 with wavy sides and a strong median keel ; ventral plate short, 

 narrow, and flat." 



