A. E. Verrill — Study of the family Pectmidm. 79 



This ribbed variety agrees with the fossil form described by Say 

 as Pecten CUntoniiis. For the northern, nearly smooth form, the 

 name given by Mighels [tenuicostata) may be retained as a varietal 

 name. 



This shell, when full grown, has the margins gaping considerably 

 at both ends, below the auricles, much as in Ainusium. But the 

 anterior sutural line is less sinuous than in most species, and the bys- 

 sal notch is 'small and not excurved. 



The gaping is less marked in the young shells, but is evident even 

 in those of small size (pi. xvii, fig. 5). The young shells show three to 

 five pectinidial teeth, but these are usually obsolete in the adult. 

 Transverse incisions on the hinge-plate are evident in the young shells. 



The muscular and pallial scars are rather complex and unlike in 

 the two valves (pi. xxi, figs. 2, 2a), but part of them are usually only 

 faintly marked, even in large specimens. The internal surface has a 

 peculiar subnacreous luster and a crystalline structure, somewhat 

 like that often shown on frosted window glass, or that on the sur- 

 face of tin-plate, after heating. 



The lai-ge shells, about 6 inches in diameter, still retain the habit 

 of swimming, though often partly covered with barnacles, hydroids, 

 bryozoa, sponges, etc., but they doubtless swim much less actively 

 than do the young ones, which are very lively. Still, I have often 

 seen the large ones leap out of buckets of sea-water in which they 

 had been placed for conveyance. The adults apparently do not ordi- 

 narily form a byssus, but there is no evidence that they are unable 

 to do so, if necessary. 



The foot of this species (pi. xx, fig. 8) has been described on page 

 69. The palpi (pi. xxi, figs. 1, la) are large and broad, triangular, 

 broadly attached at the bases which run back so as to embrace the 

 anterior ends of the gills ; their apposed surfaces are strongly trans- 

 versely ribbed. 



The oral tentacles are very large and complex, arborescently much 

 branched, so that when the branches are contracted in alcohol the}^ 

 appear somewhat like the heads of cauliflowers. The branches are 

 short and crowded and more or less webbed together, while the lat- 

 eral tentacles are attached by webs to the bases of the palpi. There 

 are five groups or clusters of these tentacles ; the two larger pairs 

 are lateral and anterior to the mouth ; the odd one, which is sim- 

 ilar but rather smaller, is in the median line behind the mouth. 



The i^allial tentacles and ocelli are very numerous {\A. xx, figs. 

 7, 8a). The ocelli differ more or less in size ; they are separated by 



