48 A. E. Verrill — Study of the family Pectinidm. 



to a larger foot, accompanied by a muscular development for creeping 

 about, and the formation of a definite byssal gland and groove for 

 more secure, but temporary, attachment. 



But the power of forming such a byssus does not imply a loss of 

 swimming habits, for we find that, at the present time, many of 

 those species that can swim most rapidly have also the power of 

 forming a byssus very quickly when they wish to rest, by attaching 

 themselves to seaweeds, etc. (See notes on Cyclopecten orbicularis, 

 p. 72.) These two coincident habits are particularly noteworthy in 

 the case of the smaller and more active forms of Pectinidse, such as 

 Palliolwn and Cyclopecten, and in the young of the larger forms, 

 such as C. irradians. This fact tends to confirm the conclusion 

 that the early Pectinidse had similar habits.' The development of 

 large, strong, or thick, ribbed and fluted shells took place later in 

 geological time and was undoubtedly accompanied by more or less loss 

 of swimming powers, just as the young of such species at the pres- 

 ent time lose more or less of their swimming habits as they grow 

 older and develop thicker and strongly ribbed shells. This loss of 

 swimming power may, or may not, be accompanied by a loss of the 

 power of forming a byssus. In some cases, like Hinnites and Hemi- 

 pecten, it is followed by a permanent attachment of the shell to a 

 solid object. The true Pectens seem to lose the byssal organs 

 when adult, and to depend upon the weight and the form of the 

 shell for safety. 



In general, those species that are best specialized for swimming 

 have a broadly rounded, symmetrical, and compressed shell, fre- 

 quently with thin, nearly smooth valves, but generally strengthened 

 by corrugations, undulations, external radial ribs, or internal Iir?e or 

 flutings. 



Species that swim but little when adult often have a high and 

 narrow form, with the auricles oblique and usually unequal, and the 

 byssal notch is often highly developed, Avhile the shell itself may 

 become oblique and unsymmetrical, or heavy and thick, with strong 

 ribs and gi'ooves. 



Most species swim well when quite small, but many lose this 



' The existence of a small gaping of the margins below the auricles at each end, 

 seems to be due to the swimming habit, for the jets of matter are mainly ejected 

 from these places (see description by Dr. Jackson). But it is difficult to ascertain 

 whether the fossil species were gaping at these places or not. In many living species 

 the gaping is very slight, but in those that are active swimmers it is often considera- 

 ble. (See C. opercularis^ C. Clintonius, Amusium.) 



