A. M Yerrill — Study of the family Pectinidce. 77 



Chlamys (^quipecten) irradians (Lam.) 



Pecten irradians Lam., Anim. sans vert., ed. 1, 1819; ed. 2, vol. vii, p 143. Gould, 

 Invert. Mass., ed. 2, p. 199, fig. 496, Verrill, Invert. Vineyard Sd., etc., p. 401 [695], 

 pi. xxxii, fig. 238. Rathbun, Fishing Industries of the U. States, sect. 1, vol. i. p. 

 509, pi. 255, fig. 8; IngersoU, op. cit., sect. 5, vol. ii, pp. 565-581, 1887. Jackson, 

 Phylogeny of the Pelocypoda, Mem. Boston Soc. Nat. Hist , vol. iv, pp. 333-350, cut 

 37, pL xxvii, fig. 9, pi. xxviii, figs. 1-10, 12, 13, 1890 (young). 



Pecten concentricus Say, Journ. Acad. Nat. Sci. Philad., vol. ii, p. 259, 1822. 



Plate XVL fig. 6. Plate XVIII. figs. 1-5. Plate XX. figs. 1-4, 6, 6a. 



Some of the young stages of this species have been figured by Mr. 

 Jackson in the work quoted above, and several of his figures have 

 been reproduced on our plate xx (figs. 1-5). We also give new 

 figures of some of the more advanced stages on plate xviii, as well 

 as a figure of the nuclear shell (fig. 1). These figures illustrate well 

 the changes that the young shells undergo. Fig. 2 of pi. xviii 

 shows three successive stages as indicated by prominent lines of 

 growth. Even in the latest of these stages the posterior auricle is 

 but little differentiated, but pectinidial teeth are already developed. 

 This figure also illustrates well the origin of the radial sculpture. 

 No camptonectes sculpture has been noticed at any stage. Radial 

 ribs begin to appear on the right valve when 2™™ in diameter, and 

 on the left valve when about rs™"^. 



The transverse incisions of the hinge-plate are usually very dis- 

 tinct in shells less than 20°^"^ in diameter, and often persist in the 

 adult. Occasionally examples are found that show them with 

 unusual distinctness. A specimen in which they are very well 

 developed is figured- on pi. xvi, fig. 6. The valves gape a little below 

 the auricles and at the ends of the auricles, to which the tentacles 

 and ocelli extend. 



The foot (pi. XX, fig. 6) in alcohol is obliquely turned to the right ; 

 it is somewhat enlai-ged at base, with a deep byssal slit extending 

 about half its length, beyond which it is contracted somewhat, and 

 slightly enlarged at the end, which is divided into two lobes by a 

 short median groove. The labial palpi are broadly triangular, 

 strongly grooved on the apposed surfaces. The oral tentacles are 

 large and consist of numerous contorted lamellae, much webbed 

 together, and united with the bases of the labial palpi. There 

 appear to be two (or two groups) on each side and somewhat in front 

 of the mouth. The gills are large and of the type usual in this 

 family. The pallial tentacles are very numerous and very unequal 

 in several rows. Two or more of the larger ones correspond to 



