122 PROCEEDINGS OF THE ACADEMY OF [1888« 



from a coinmou or what might be called a central type, and that a 

 divergence from some cause set in, producing on one side the Ostrea,. 

 and on the other the AsperglUum. 



As regards the whole class, he said there is no doubt, in his mind 

 at least, that it is a degenerate one. ^lany anatomical and embryolog- 

 ical facts, as Avell as their life habits, point to this, it being an acknow- 

 ledged fact that fixed or stationary animals are as a rule deg-enerate. 

 The loss of the head in all adult forms, the presence of eyes in the 

 head area of some free swimming embryos, and their later total dis- 

 appearence, are some facts that point unquestionably to the degenerate 

 condition of the whole group. 



As to the facts of geology pointing to this conclusion, he quoted 

 from Prof. Heilprin's work on the "Distribution of Animals," p. 265. 

 "Almost everywhere, the Cephalophora, or head-bearing mollusks, 

 antedate by one full period the Acephala, or headless forms, which 

 indisputably represent a lower grade of organism." By considering 

 the group as degenerate, the conditions of the case are answered, for 

 the Lamellibranchiata certainly came off* from the Gastropoda, after 

 the latter had become W'cll established, as the anatomical and embry- 

 ological facts show, and the geological evidence would seem to 

 indicate this to be the case. 



Assuming then, that the Lamellibranchiata have come off" from 

 the Gastropoda, Dr. Sharp then considered what was the form of the 

 primitive type. It probably had a more or less developed foot, an 

 organ that is present in all the Gastropoda, that it possessed gills on 

 each side of the foot, that the mantle edges were separate and that 

 two adductors were present of about equal size. This type has sur- 

 vived to the present day and, according to Lankaster (Art. Mollusca, 

 Brit. End. jd. 685), is represented by forms like Nucida and Trigo- 

 nia. The former belongs to the family Arcidae {Glaus) which 

 is the oldest type that we know of, being found in the Silurian and 

 Devonian. The shells of this' family are equal ; the adductor mus- 

 cles of the same size, the mantle free, not being closed to form 

 tubes like a siphon, foot well developed. The fulcrum of the shell 

 is about equi-distant from the adductoi's. In following one branch 

 from this toward Ostrea, it is found that one muscle, the anterior, 

 gradually gets smaller, as is the case in Mytilis, and exceedingly 

 small in Pinna, until in Ostrea but one muscle is present. From 

 the fact that in this advance the animal becomes more and more 

 fixed first by a secretion of the foot, the byssus, then by the shell it- 

 self, the foot gradually is less and less used as an organ of locomo- 

 tion, until it entirely disappears in Ostrea. The retractor muscles of 

 the foot, now practically useless organs, are however, still present. 



The loss of one adductor muscle can probably be referred to me- 

 chanical causes. In studying the change of relation of the fulcrum 

 to the adductors, he found that as the fulcrum moved forward (an- 

 teriorly) it increased the distance from the posterior, and lessened the 

 the distance from the anterior muscle. As this took place, the muscle 



