38 DESCRIPTION OF NEW 
Unio Incatustanus. Pl. XXIV. Fig. 41. 
Testa levi, transversd, inflata, cylindracea, posticé acuto-angulata, e natibus postice bilineatis; valvulis 
subtenuibus; natibus subprominentibus ; epidermide tenebroso-olivacea, striata; dentibus cardinalibus pre- 
longis, laminatis et in valvuld dextra solum duplicibus; lateralibus longis, laminatis, rectis; margarita 
cxruleo-alba et valde iridescente. 
Shell smooth, transverse, inflated, cylindrical, acutely angular behind, with two lines posteriorly from the 
beaks; valves rather thin; beaks rather prominent; epidermis dark-olive, striate; cardinal teeth very long, 
lamelliform, straight and in the right valve only double; lateral teeth Jong, lamelliform, straight; nacre bluish- 
white and very iridescent. 
Hab. Siam, Thomas R. Ingalls, M. D. 
My cabinet and cabinet of Dr. Ingalls. 
Diam. .7, Length .8, Breadth 1.9 inches. 
Shell smooth, very transverse, inflated, cylindrical, acutely angular behind, obtusely 
rounded before, with two minute, slightly elevated lines running from the beaks towards 
the posterior margin, on the posterior slope flattened, near the basal margin slightly com- 
pressed; substance of the shell rather thin; beaks rather prominent, submedial; ligament 
short and rather thick ; epidermis dark-olive, dull, striate over the whole disk, without rays; 
umbonial slope obtusely angular; cardinal teeth very long, lamelliform, straight, and in 
the right valve only, double; lateral teeth long, lamelliform, straight and double in the left. 
valve; anterior cicatrices confluent; posterior cicatrices confluent; dorsal cicatrices placed 
on the plate immediately over the centre of the cavity of the beaks; cavity of the shell 
deep and rounded; cavity of the beaks rather shallow and rounded; nacre bluish-white, 
pearly and iridescent. 
Remarks.—This very interesting shell was sent to me by Dr. Ingalls, of Greenwich, N.Y. 
It was the only new one among several he received from Siam. ‘To his kindness for this 
and other species, I am indebted, and I dedicate it to him. The form is very unusual, and 
assimilates to the U. Caillaudii, Fer., from Egypt. The colour is of the same dull olive. 
It may be distinguished in the exterior by its being less cylindrical and more angular 
behind; and in the interior, by the peculiar form of the cardinal teeth, which are long and 
lamelliform and double in the right valve, while in the Cutllaudii they are short, erect, and 
double in the left valve. The beaks are also much more medial. The nacre is of an 
uncommonly fine pearly character, and in this specimen it is delicately striate from the 
cavity of the beaks towards the margin. In the description, it may be observed that I 
have stated the anterior cicatrices to be confluent, a rather unusual character in the 
Uniones. Of the smaller under cicatrix there is no appearance in this specimen, but it 
may be embraced in the large one. There is, however, a very distinct cicatrix above the 
large one. It is much to be regretted that the soft parts were not preserved, as the 
peculiar cardinal teeth and muscular cicatrices indicate a different disposition of the mus- 
cular fibres of the adductor and retractor muscles. The enlargement on the posterior 
portion of the disk towards the basal margin, indicates the female character of this 
specimen. 
