462 
THE GEOLOGIST. 
spines. But I must refer the reader to Piofessor Hall's admirable 
work, where several species have been carefully described. 
Of the genus or sub-genus (?) Uncites but one or two species have 
been discovered, and its interior character has not been made out to 
my entire satisfaction. The shell is oval, elongated ; the beak of the 
ventral valve long, tapering, and incurved at its extremity, hollow and 
truncated in young specimens, by a small oval foramen ; no true area ; a 
1 xrge concave deltidium partly surrounds the aperture, and extends to near 
the cardinal edge ; the sides of the beak of the larger, as well as of the 
smaller, valve become sometimes considerably deflected inwards, pro- 
ducing deep lateral elongated depressions or pouches, opening exter- 
nally, but- not communicating with the interior. The muscular 
impressions have not been described, and the existence of spiral pro- 
cesses has been recorded upon the evidence of but a single imperfect 
specimen. 
The third group is at present limited to a single genus, to which 
Dalman applied the name Atrypa, from having supposed that his shell 
was deprived of a foramen ; but, although the aperture is in some speci- 
mens concealed by the incurvation of the beak, a small round opening, 
separated from the hinge-line by means of a deltidium, may often be 
observed not only in youug shells, but in many that have attained their 
fullest development. The term Atrypa is, therefore, a misnomer, as 
well as that of Aihyris, and, if retained, should be looked upon as a 
simple. denomination, casting aside the derivation. On the Continent 
the term Spirigerina is used, but many prefer to retain the more 
ancient denomination. The species which compose this genus have been 
well studied, so that the dispositions of the interior have been almost 
completely discovered. The shell is circular, elongated, or transverse, 
smooth or variously ribbed, and imbricated by numerous squamose lines 
of growth, which are at times considerably produced beyond the margin 
in the shape of tubular spires or foliaceous expansions. In the interior 
of the larger valve, at the base of the teeth, a semicircular ridge curves 
on each side, forming a saucer-shaped depression, open in front. This 
space is entirely filled up with muscular impressions ; the adductor or 
occlusor forms a small longitudinally oval mesial impression, and on 
either side two larger scars wore produced by the cardinal or divaricator 
muscles, which are also flanked by the ventral pedicles or adjuster 
muscle in a very similar manner to what we find in Terebratula, 
