OF CONCHOLOGY. 



45 



Terebratulidce and in some forms, as among the brachiopods, 

 are wanting. 



It is noticeable that if a diagram be made of a chiton with am- 

 bient branchiae, in the position which it assumes when removed 

 from its station on a rock, that the position of the oviducts, 

 branchiae and heart resembles that of the same structures in the 

 molluscoidea, but with the position of the heart reversed in rela- 

 tion to the ganglion, indicating the position of the group to be 

 among the typical mollusks. I observe a character, which, in 

 one sense, may be termed degradational ; that is, that there ap- 

 pears to be a sort of fading out of the characters of the anterior 

 part of the body, so that the oviducts, heart, and in many cases 

 the branchiae, are more or less concentrated toward the posterior 

 end of the body. It has been stated that the branchiae proceed 

 from the tail toward the head, but this appears to me to be in- 

 correct. In all the chitons I have examined, the branchiae point 

 toward the tail, even when wanting in the cephalic region, and 

 there is always a space entirely without branchiae at the posterior 

 extremity. In the chitons with ambient gills, no such space 

 occurs in front, the arteries and nerves are most emphasized 

 anteriorly, so that I am forced to the conclusion that the bran- 

 chial cordon really springs from the cephalic region as in the 

 limpets, though very different in character from theirs. 



The annulated mantle filaments of Lima recall the similarly 

 annulated setae of some brachiopods, though no homology is in- 

 tended to be indicated by this observation. (See Forbes and 

 Hanley, Brit. Moll, i, pi. R.) 



Sub-type Molluscoidea. 



Mollusca having one principal subcesophageal ganglion or pair 

 of ganglia ; provided with an atrial system ; haemal vessels more 

 or less imperfect ; heart provided with accessory " pulsatile 

 vesicles," imperfect, or entirely absent ; when present, situated 

 on the same side of the intestinal canal as the ganglion, though 

 through the flexure of the intestine, it is often on the opposite 

 side of the body, and thus apparently, but not really, on the op- 

 posite side of the intestine ;* respiratory apparatus about, and 

 to a greater or less extent surrounding, the mouth or oral aper- 

 ture ; reproduction by gemmation or ova ; breathing water only. 



While the classes included in the above diagnosis possess the 



* Those cases which form exceptions to this general rule also exhibit 

 abnormal relations, in position, of the other parts, and are to the normal 

 forms as the sinistral varieties of Buccinum are to the typical dextral 

 form of the particular species. 



