462 



THE GEOLOGIST. 



spines. But I must refer the reader to professor 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 

 lirge 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, m^ay often be 

 observed not only in young shells, but in many that have attained their 

 fullest development. The term Atrijpa is, therefore, a misnomer, as 

 well as that of Athyris, 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 were 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^ 



