A. E. Verrill — SUaly of the family Pectinidm. 49 



power, more or less, when adult, especially in the ease of the one- 

 sided Pectens and those species that have very oblique shells, like 

 certain species of Chlamys, for example C. madreporarum^ which 

 lives, when adult, in cavities between the branches of corals from 

 which it cannot escape. This shell becomes very oblique and has 

 very unequal auricles. Hinnites has gone farther in this direction, 

 and has become attached and irregular when adult. 



C. Clintonius, when it becomes very large, seems to be nearly 

 sedentary and is often heavily covered with barnacles, ascidians, 

 bryozoa, etc., but I have never found it with a byssus when adult. 

 It probably retains more or less power of swimming through life, 

 and seems to be migratory in its habits, changing its station accord- 

 ing to the seasons. The same is true of Chlamys irradians and its 

 allies. Rut C. (Jlintonius, like Amushim, has the foot rather large 

 and divided at the toj) into two lobes by a deep fissure, so that the 

 lobes can be spread apart in the form of a terminal disk. It is prob- 

 ably used as a pushing organ.' By means of thig organ these 

 species can probably push themselves slowly about when necessary 

 to change their positions. 



The teleological reasons for the development of ribs, flutings, and 

 other forms of sculpture on the shells have been discussed elsewhere 

 (pp. 42, 43). It is only necessary to state here that as the strongly 

 ribbed and fluted conditions naturally and necessarily succeed the 

 simple and fine ribbed stages, during growth, so in geological time 

 the strongly ribbed genera succeeded the simpler and thinner forms, 

 though many of the latter persist at the j)resent time. But still, 

 strongly ribbed forms of Pecten, Chlamys, JSTeithea, etc., had alreadj^ 

 become well-developed in mesozoic times, and Lyropecten, remarka- 

 ble for the strength of the ribs, apj^eared early in tertiary time. 

 But some of the modern, thin, smooth forms have probably descended 

 from ribbed species, by the gradual reduction of the ribs.^ It is 

 doubtful whether any generic or subgeneric group of Pectinidse has 

 been evolved since the Eocene. Even in the Cretaceous, nearly all 

 the existing generic and sectional groups were in existence, together 

 with a few that are now extinct. 



^ Dr. Dall has described this organ as a terminal "sucker," but it is doubtful if it 

 can be used for adhesion. 



2 This appears to be the case with Placopecten and Lissopeden, in which the ribs 

 have become nearly obsolete, though they are in other respects nearly allied to 

 Ghlamys. 



Trans. Conn. Acad., Yol. X. June, 1897. . 



4 



