No. 407.] PECTEN IRRADIANS LAMARCK. -> 865 
were examined by me, I found no exception to the condition 
of being notched on the right ear. Jackson ('90) found no 
exception to the normal condition, and states that Professor 
Hyatt examined over three hundred specimens of P. irradians, 
all of which lay on the right side. There is, accordingly, a 
wonderful constancy in the tendency to lie on the right side ; 
and this tendency is characteristic of many related genera, 
such as Pinna, Spondylus, Plicatula, Hinnites, and Anomia. 
The constancy in the position of the notch is referable to a 
constancy in the position of the byssus gland. This is nor- 
mally laid down in development as a pair of glands, right and 
left. Apparently only the right gland persists, or else perhaps 
both glands are united on the right side. Whatever the mor- 
phogenetic process which determines the position of the byssus 
gland, it is a remarkably constant one. 
The shell of P. irradians exhibits on its outer surface 
five areas : First, a middle one, characterized by large prom- 
inent ridges, alternating with furrows. Since these are made 
by foldings of the mantle, the outer ridges appear as grooves 
on the inner surface, and the outer grooves as inner ridges. 
Second, a pair of “ear” areas characterized by fine linear ele- 
vations which do not correspond with marked grooves on the 
inner face. Third, a pair of triangular areas, lying between 
the middle areas and the ear areas, and transitional between 
them. These may be called the “transition” areas. They 
bear indistinct radial thickenings. These areas are bounded 
laterally by the ear areas. On the right, or notched, side of 
the right valve the boundary is distinct ; on the left side of the 
right valve and on both sides of the left valve the lateral limit 
of the transitional area is determined by the line where the 
ear begins. But this is not a very precise line. The transi- 
tional areas are bounded mesially by the middle area. The 
limit may be defined as the line where an internal groove 
corresponding to an external ridge first appears. Since the 
external ridges on the shell are so often obscure, I early aban-. 
doned the attempt to count them. I then noticed that the 
internal grooves are much more precise than external ridges ; 
