598 William Patten 



Pecteìi operctäaris. Otlier species, as P. piisio, P. carms^ P. flexuosus, 

 P. inflexiis and P. testete, were also studied, but, in ali except P. pusio, 

 the differences were imappreciable. 



The foUoAviug description, with the exception of one or two remarks 

 concernine P. j^usio, refers entirely to P. opercularis. 



The mantle edge of yoimg- Pectens has the three characteristic folds 

 well developed. In the adnlt, these folds are enlarged and complicated 

 by the presence of secondary ones, and several rows of tentacLes. The 

 walls of both the Ophthalmie and shell folds are specially thickened, 

 and ciliated on the shéll side. The former, in P. opercularis^ is short 

 and thick, its free extremity being divided into two thick-walled and 

 ciliated lobes; in P. pusio it is unequally forked, the outer division 

 being much longer and thicker than the inner. 



The pigment in the youngest specimens, 1 mm long, is entirely con- 

 fined to the eye spots. In larger ones of 2 mm, the velum is covered 

 with light yellow or red blotches, in which are many large, scattered 

 ommatidia, usually single, but often united into pairs. They form ir- 

 regulär black spots, consisting of several darkly pigmented cells sur- 

 rounding a centrai colorless one, These isolated ommatidia are larger 

 and more conspicuous than any I have ever seen. In the adult, they are 

 less prominent and may disappear entirely. Between the base of the 

 Ophthalmie fold and the velum is anarrow pigmented band, extending 

 the whole length of the mantle, and, at places, deepeued into lougitu- 

 dinal grooves, from the bottom of which are often developed the hypo- 

 dermie thickenings, which constitute the rudiments of the eye. The thin 

 branchial wall of the Ophthalmie fold, usually colorless, with bere and 

 there a minute, yet black pigmented pit (fig. 2 y),\% seldom provided with 

 cilia, which are principally confined to the two thickened, terminal lobes 

 and the opposite face of the fold. The pigmented pits consist of minute, 

 deeply pigmented cups, sharply circumscribed, and not to be confounded 

 with simple, irregularly shaped pigment spots upon the level surface of 

 the Ophthalmie fold. Ngt more than three or four cells, completely iilled 

 with black pigment, take part in the formation of these pits, which, in 

 their Position and general appearance, recali the invaginated eyes of 

 Arca. The colorless cells, so universally present in the latter, could not 

 be detected ; but in spite of their absence (probably due to haviug es- 

 caped notice on account of their minute size and the diffìculties of Ob- 

 servation), I consider that these transitory pigmented cups 

 are homologous with the invaginated eyes of Arca. 



In cross sections of the very youngest specimens (1 mm long), may 



