46 A. JEJ. Verrill — Study of the family Pectinidce. 



other often becomes obsolete in the adult. The Pectinidse all retain 

 both these structures in a decidedly primitive form. The ligament 

 is thin and but little differentiated, and occupies a straight, narrow 

 marginal groove along the whole of the hinge-line. The resilium is 

 wedge-shaped or triangular, nearly central, with its apex joining the 

 ligament. This seems also to be nearly the condition in the youngest 

 shells of this group that I have been able to examine. The very 

 early stages figured by Dr. Jackson seem to indicate the same thing. 



The post-veliger shells, " nepionic stage," in all the genera that I 

 have examined, are nearly smooth, and have, at first, only slightly 

 angulated dorsal margins, indicating the origin of the auricles, 

 but the auricles develop rapidly and soon become sufficiently evi- 

 dent, the anterior one developing more rapidly and showing a byssal 

 notch very early. In some species, like C vitrea, the posterior 

 auricle always retains its early undeveloped form. 



In most species the form of the shell, characteristic of the family, 

 is developed before the larval shell or "spat" is 1™"" in diameter. 

 When about H to 2"'™ in diameter the characteristic sculpture usu- 

 ally begins to appear. In most forms this consists at first of a 

 number of small, straight radial riblets on both valves. These may 

 or may not be accompanied by a peculiar, divergent vermiculation 

 or "camptonectes-sculpture." When the latter appears at all, it 

 either slightly precedes, or is simultaneous with, the radial riblets. 



A few species, like P. simile (PI. xvii, figs. 8, 8a), do not distinctly 

 develop either of these forms of radial sculpture at any stage, but 

 seem to retain through life the simple condition of the smooth or 

 radially striated larval shell, as well as much of its form, with little 

 alteration. Others retain the " camptonectes-sculpture" and fine 

 riblets without much change ( C. striata). Hence we may conclude 

 that in such groups as Hyalopecten, Palliolum, and Camptonectes we 

 have survivals of very primitive or archaic Pectinidfe. This is also 

 indicated by the very feeble differentiation of the posterior auricle, 

 so noticeable in C. vitrea (PI. xviii, fig. 6, 7, 10, 11) and in 

 P. simile (PI xvii, fig. 8). In these species the posterior auricle 

 retains the form that it has in the young spat of Chlamys irradians 

 (PI. XX, fig. 3), and other more highly specialized species. 



It is possible that the power of swimming, so well developed in 

 young Pectinidae, was acquired by the early fossil forms, even in 

 the palaeozoic ages. Possibly some of the early small forms of this 

 o-roup developed the power of swimming b}^ the sudden closure of 

 the shell before they entirely lost their ciliated velum, so that they 



