74 AKCAD.E. 



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 point 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 Area, 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, being throughout as straight as in Area ; the 

 impressions of the adductor muscles scarcely diifer in the two, 

 and there is in both the same dull scabrous irregularly nodose 

 aspect of the inside of the valves. The ventral gape in the 

 regular specimens of A. tetragona is similar in Galeomma. 

 The character of the striae or minute ribs is the same as in 

 Area lactea, except that in it the bifurcations are seen inter- 

 nally, whilst in Galeomma they are external. With high 

 powers, the oblique, though nearly obsolete teeth may be 

 observed on the ligamental line, and the internal 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 Arcadce. 



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 Arcadae, are the 

 regular equidistant frosted-white eminences or ocelli, which, 

 except in colour, entirely resemble those in the typical Area ; 

 and as there are no other genera but the Arcae 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 



