No. 407.] PECTEN [RRADIANS LAMARCK. 873 
A second class of modifications consists in the formation 
of striations on the ribs. These striations are thickenings of 
the shell in radial lines. They may in some cases become so 
pronounced as to form ribs at the periphery of the shell. 
These striations I regard as secondary, both because they are 
clearly something added to the simple rays which we have in 
Pecten, because the young stages of Pecten show no such stria- 
tions, and because in the young stages of striated shells the 
strize are absent. Consequently these striated species are more 
modified than irradians. 
A third class of modifications is that of smooth or nearly 
smooth shells. Of this condition our P. magellanicus (= clinto- 
nius) is typical. Although the ancestors of Pecten were smooth- 
shelled, the magellanicus modification is by no means ancestral, 
for if, as Verrill (99, p. 78) states, “when about 3-4 mm. in 
length it develops small, regular, raised ribs over the whole 
surface of the upper valve and usually at both ends of the 
lower one,” this shell must have been derived from ribbed 
ancestors. So, too, in P. glaber of Smyrna and P. danicus of 
Scotland we have the process of obliteration of ribs going on 
with the formation, apparently, of a few large secondary crena- 
tions.! In all cases we start with a ribbed form like irradians. 
Let us consider, finally, the relation of irradians to the other 
species of Chlamys in respect to the number of ribs. The 
data for such a comparison can be got from an important lot 
of countings made by Dall (98) and from some determinations 
of my own made on shells in the collection of the Field Colum- 
bian Museum. I give only modes (or, in the absence of suffi- 
cient data, the ranges) for the /z/7 valve. In some cases 
external ridges are given because these are often alone avail- 
able and Dall has counted ridges only. In other cases I give 
internal grooves ; these are exclusively derived from my count- 
ings. Since the extreme lateral external ridges usually have 
no internal grooves, the groove numbers run one or two lower 
than the ridge numbers. 
1 An insufficient amount of material requires me to put forward this explana- 
tion with some reserve. 
