44 A. M Verrill — SUidy of the family Pectinidae. 



then there would be but little surface presented to the waves. The 

 species of this group usually live in rather shallow water, within the 

 limits of wave-action. It is usual for oysters and other attached 

 forms to have the attached valve deepest. When the shells of these 

 one-sided Pectens and others of similar form are dropped into the 

 water they generally sink with the flat valve uppermost, so that this 

 form may be useful in keeping them " right side down," now that 

 they have acquired a structui'e that requires them to lie on the right 

 side, but this will not explain the first origin of the form, which was 

 probably due to the gradual loss of swimming habits. 



These one-sided Pectens seem to be rather sessile, as compared 

 with most of the other groups, and certainly the great thickness and 

 weight of the shell, and its special form, do not seem to be adapted 

 to active swimming habits. On the other hand, the byssal organs 

 for attachment are not very well developed, so that the adult shells 

 probably rest upon the bottom unattached, and move about by 

 means of the foot. 



That they do not have the swimming habit well developed is also 

 indicated by the unusually tight closure of the valves at the base of 

 the auricles, where the expulsion of the water takes ^lace during the 

 act of swimming, in this family. The right valve is strongly 

 excurved at the byssal notch, and the left valve is strongly bent 

 inward at the corresponding part, so as to fit the marginal notch very 

 completely, leaving at most only a narrow passage for byssal threads. 

 The form of these shells is poorly adapted for swimming, for if cur- 

 rents of water should be expelled in the usual way, from the sub- 

 auricular margins, the currents would naturally be forced upward, 

 and out of the concave lower valve, and thus the reaction would be 

 strongly downward, so that the shell would not be raised from the 

 bottom. 



In those species that are able to rise to the surface and swim 

 actively about, the left or upper valve is always the more convex, 

 and therefore the expelled currents of water must be directed more 

 or less downward, so that the reaction forces the shell upward as 

 well as backward. 



In the typical one-sided Pectens the foot is pretty well developed, 

 and there is a strongly marked and usually double scar on the left 

 valve, where the pedal muscle is attached, above the scar of the 

 adductor muscle. It is probable, therefore, that they can use the 

 foot effectively for moving about, even when adult. 



