44 PROCEEDINGS OF THE MALACOLOGICAL SOCIETY. 



If Haller's figures are studied the oesophagus will be seen to intervene 

 between the coecum and intestine and the former does not occupy 

 the same position as it does in Pterocera. 



With these reservations we may now consider these structures 

 from a wider standpoint. We first of ail see that the style, either 

 enclosed in a sac or free, is a fairly widely distributed feature in 

 Prosobranchs, though it is probably limited to the Docoglossa, 

 Ehipidoglossa, and Tsenioglossa. We next see that there is an 

 extraordinary parallelism between the Gastropoda and Lamelli- 

 branchia in the ultimate separation of the style-sac from the pylorus 

 and the occurrence of intermediate types in which the separation is 

 incomplete. Thus Yoldia, Area, and Mytilus on the one hand, and 

 Fissurella and Cyclostoma on the other, represent the stage when the 

 style is either free in the pylorus, oi a specialized part of the latter 

 is still in wide communication with the intestinal part. Next, we have 

 Modiolaria among Lamellibranchia and Paludestrina and Hypsohia 

 among Gastropoda in which the communication is very much 

 restricted. Finally, we have Pholas and Donax on the one hand, 

 Adeorhis. Typhobia, and Pterocera on the other, in which the style-sac 

 is fully differentiated and completely separated from the intestine. 



We have hitherto spoken as though the style-sac was differentiated 

 off the pyloric part of the intestine. The reverse possibility is 

 suggested by Ghosh.® According to his view it is just as likely that 

 the style-sac evolved as an outgrowth from the stomach 

 independently of the pylorus, such- a separate style-sac being 

 " present in the ancestral forms before the evolution of the present 

 class " (I.e. p. 73). Such a suggestion deserves serious consideration, 

 though I do not consider it indicates the more likely course of 

 events. In the Gastropoda the evidence seems to favour the view 

 that the course of development was from original imity with the 

 pylorus' to subsequent separation. Thus we have a style only in 

 Fissurella and Cyclostoma, while among the rest of the Tsenioglossa 

 we have the less specialized Paludestrinidse showing a partly 

 differentiated sac and the more specialized Pterocera and Turritella 

 with the sac separated. This part, of the argument conceivably 

 might be met by pointing out that Adeorhis, which has a separate 

 style-sac, is considered to have affinities with the Rissoidee, which 

 are again fairly akin to the Paludestrinidae. 



In the Gastropoda the morphological status of the various 

 suborders and families is fairly clear, and one may be tolerably 

 certain as to the position of a form used in such an argument as the 

 above. With the Lamellibranchia, however, the matter is otherwise. 

 We know that some of the Protobranchia are certainly primitive, 

 but beyond that it is very difficult to be absolutely sure that the 

 taxonomic position assigned to an animal is any index of its real 

 morphological status. As a consequence, generalizations about forms 

 exhibiting modifications of a certain character are apt to be very 



