48 PTEROPODA. 



sisting of a dorsal and ventral plate united, or forming a 

 spiral. 



The Pteropocla are divided into two orders, termed Thccoso- 

 mata and Gymnosomata ; the former characterised by possess- 

 ing an external shell and an indistinct head ; the latter by 

 being devoid of a shell, and by having a distinct head, with 

 fins attached to the neck. 



The Gymnosomatous Pteropods, in which there is no 

 shell, as a matter of course, are wholly unknown in the 

 fossil condition. The Thecosomatous Pteropods, in which 

 there is a shell, are divided into two families — the Hyaleidce 

 and Limacinidce. The latter comprises forms in which there 

 is a small spiral shell, which is sometimes provided with an 

 operculum ; but it is unrepresented in a fossil state. The 

 former family comprises forms in which the shell is sym- 

 metrical, straight or curved, globular or needle-shaped, and 

 it is represented by a considerable number of fossil forms, 

 most of which are extremely unlike any known living ex- 

 amples of the class, being often of comparatively colossal 

 dimensions. The fossil forms mostly belong to the genera 

 Hyalca, Cuvieria, Cleodora, Hyolithes [Tlucct), Pterotheca, Ten- 

 tamdites, and Comdaria ; but other less important types are 

 known to have existed in past time. Of the above-men- 

 tioned generic forms, the first three are well represented at 

 the present day by living forms. The remaining four are 

 almost exclusively Palaeozoic, Comdaria alone sur\dving into 

 the earlier portion of the Mesozoic period. Not only is 

 this the case, but the forms in cjuestion all commence their 

 existence in the Lower Silurian or Upper Cambrian, and 

 only Hyolithes and Comdaria transgress the upper limit 

 of the Devonian rocks. Lastly, almost all these forms 

 are of comparatively gigantic size, and they differ in many 

 respects from living forms. 



Among the fossil members of the class, the largest genera 

 are Tentamdites, Hyolithes (Thcca), Comdaria, and FtcrotJura. 

 These are also the most widely distributed types, and the 

 ones with the longest vertical range. Taken as a whole, the 

 Pteropods attain their maximum as fossils in the Lower 

 Silurian rocks : they diminish gradually in number towards 



