126 Miscellaneous. 



tive that they only cover an exceediiagly small area of the animal's 

 body, the siphon is enormously developed, and it is protected by a 

 secretion of carbonate of lime, in which the shells are immovably 

 imbedded ; the mantle is closed throughout its entire length, except 

 at the anterior end, wbere there is a minute opening, and at the 

 mouths of the two siphoual tubes. 



His object in making the communication was to prove that these 

 two very marked and different types of Lamellibranchiata arose 

 from a common, 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 Aspergillum, 



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

 at least, that it is a degenerate one. Many anatomical and embryo- 

 logical facts, as well as their life-hahits, point to this, it being an 

 acknowledged fact that fixed or stationary animals are as a rule 

 degenerate. The loss of the head in all adult forms, the presence 

 of eyes in the head-area of some free-swimmiug embryos, and their 

 later total disappearance, 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 ofl' from the Gastropoda, after 

 the latter had become well established, as the anatomical and 

 embryological 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, it possessed gills on 

 each side of the foot, the mantle-edges were separate, and two 

 adductors were i)resent of about equal size. This type has sur- 

 vived to the present day, and, according to Lankester (art. MoUusca, 

 Encycl. Brit. p. 685), is represented by forms like Nucala and Tri- 

 gonia. The former belongs to the family Arcidie (Claus), 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 

 muscles 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 equidistant from the adductors. In following one branch 

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

 gradually gets smaller, as is the case in Mytilus, 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, thei\ by the shell 



