74 ARCADA. 
It will be observed in the account of this animal, taken from 
notes made twenty years since, that the mantle is partially 
closed in front; this is the only pomt against the Arcadan 
position, as in the typical species that organ is open through- 
out the ventral range. We certainly did not then observe so 
critically as now; there may be an error; the animal requires 
further investigation, which we confidently believe will confirm 
our present views. Galeomma, by the contiguity of the beaks 
and very limited ligamental central facet or area, together 
with the minute glossy circular porcellanous nuclei of the 
umbones, is closely allied to Pectunculus,—also, perhaps, more 
so to Arca, particularly A. tetragona,—by the linear character 
of the hinge and ligament, which has the central portion semi- 
internal, and is carried on each side to the termination of the 
dorsal line, beng throughout as straight as in Arca; the 
impressions of the adductor muscles scarcely differ in the two, 
and there is in both the same dull scabrous irregularly nodose 
aspect of the imside of the valves. The ventral gape im the 
regular specimens of A. tetragona is similar in Galeomma. 
The character of the striz or minute ribs is the same as in 
Arca lactea, except that in it the bifurcations are seen inter- 
nally, whilst im Galeomma they are external. With high 
powers, the oblique, though nearly obsolete teeth may be 
observed on the ligamental line, and the mternal linear areas 
are similar to those in A. ¢tetragona and P. glycimeris. Many 
other minute similitudes may be mentioned, but enough has 
been advanced to show that the conchological indices decidedly 
fix Galeomma with the Arcade. 
Let us speak of the internal organs. The foot in Galeomma 
is byssal, and precisely of the same form and character as that 
of A. tetragona. As a last, but, as we think, very convincing 
proof of the relation of this genus to the Arcade, are the 
regular equidistant frosted-white eminences or ocelli, which, 
except in colour, entirely resemble those im the typical Arce ; 
and as there are no other genera but the Arce and Pecten in 
which the mantle of the animal is furnished with ocelli, it 
follows, from the entire difference of the position of the ad- 
ductor muscles in Pecten and Galeomma, that this species, on 
