804 DIVISIONS OF THE GASTROPODA. 



" shell " of Cymbulia is of a cartilaginous consistence, but it does 

 not correspond with the shell of the other Thecosomatous Pteropods 

 in its nature or origin. In the majority of the shell-bearing Ptero- 

 pods the shell is symmetrical, but in the Limacinidce. it is coiled into 

 a spiral. The Pteropods are all hermaphrodite, and the young pass 

 through a metamorphosis. 



The Pteropoda have been very generally regarded as a distinct 

 class of the Mollusca ; but the most recent investigations, more 

 especially those of Dr Pelseneer, prove that in the essential details 

 of their organisation they do not differ materially from the Gastro- 

 poda, of which class they should be considered as a division or 

 order. The Pteropods fall naturally into the two sections of the 

 Gymnosomata and the Thecosomata, the former devoid of any shell 

 in the adult condition, while the latter (with the exception of the 

 aberrant genus Cymbulia) possess an external calcareous shell. 

 Owing to their having no hard parts capable of preservation in the 

 fossil condition, the Gymnosomatous Pteropods have left behind 

 them no traces of their past existence, and therefore require no 

 further consideration here. On the other hand, the Thecosomatous 

 Pteropods are known to occur as fossils, though opinions differ as 

 to the true nature of many of the organisms which have been re- 

 ferred here. 



As regards their distribution in time, the Tertiary rocks have 

 yielded the remains of indubitable Pteropods, which differ in no 

 respects from existing forms, and are, in fact, largely referable to 

 existing genera. The whole series of the Secondary rocks has, on 

 the other hand, yielded no remains of organisms which can be 

 asserted to be unequivocal Pteropods. It is not, indeed, till we 

 reach the Devonian and Silurian deposits that we meet with fossils 

 which in all essential respects agree with the living Pteropods, and 

 which appear to belong to such existing genera as Styliola (Creseis). 

 It is true that these ancient representatives of Styliola have had their 

 claim to be regarded as Pteropods strongly disputed, but it will be 

 pointed out hereafter that there is at present no sufficient ground 

 for denying their Pteropodal nature. The case, however, is different 

 with the three remarkable groups of fossils typified respectively by 

 the genera Tentaculites, Hyolithes, and Conularia, the age of which 

 is exclusively or mainly Palaeozoic. These ancient types differ in 

 many respects from the typical Pteropods, and their true nature 

 and position must be regarded in the meanwhile as more or less 

 open to question. If these aberrant and problematical genera be 

 provisionally included among the Thecosomatous Pteropods, the 

 following are the principal families included in this division of the 

 order : — 



Family i. Limacinid^. — In this family the shell is twisted into 



