On the Genus Tellinomya, and allied Genera. 393 



beak on both sides. The teeth are often very minute immediately 

 beneath the beaks. The shells of this genus vary from elliptical 

 to ovate and sub-triangular forms, many of them being contracted 

 on the posterior side ; they are usually of moderate thickness, 

 though one species is very thick and strong. Some of the species 

 have a distinctly impressed lunule. The lesser muscular impres- 

 sion is often a small pit placed directly beneath the hinge line and 

 between it and the large muscular impression. 



The beaks are usually of medium size, pointed, rarely ventricose, 

 approximate or in contact, never subspiral. 



The relations of this shell are among the Arcadse, and approxi- 

 mate to the Nuc^ilse in their general characters, and to which 

 genus they have usually been referred. They differ from that 

 genus, however, in the absence of the ligamentary pit beneath the 

 beak, and in the presence of an external ligament and double 

 muscular impressions. 



It is probable that most of the Palseozoic species referred to the 

 genus JSfucula belong to Tellinomya, except those of the genus 

 JV'itcidites of Conrad, Cucullella of McCoy, which is distinguished 

 by the presence of a septum in the anterior part of each valve. 

 The place of Tellinomya may be regarded as between Nucula and 

 Nuculites. In external characters it may prove difficult to sepa- 

 rate Tellinomya from Nuculites, but the presence of the septum 

 affords nearly the same degree of difference as that between 

 Cucullella and Area. 



The species of the genus Cucullella of McCoy are cited from 

 Upper Ludlow rocks ; and the species of the genus Nuculites of 

 Conrad are, with one exception,* from the Hamilton group, or 

 rocks of the same age. We may therefore infer, with some rea- 

 son, that the shells having crenulate hinge lines, with the internal 

 septum, occur in rocks of later date, or, in other words, that they 

 do not begin their existence before the Upper Silurian or perhaps 

 the Devonian period, while the TelHnomyae occur among the 

 earliest forms of lamellibranchiate shells. 



For the purpose of comparison with Tellinomya, and as exhibit- 

 ing in some degree similar characters with that genus, as well as 

 to show the marked identity of description in these two genera, I 

 give below the generic characters of Nuculites and Cucullella, 

 from the descriptions respectively of M. Conrad and Professor 

 McCoy : 



* This exception has been found to be destitute of teeth in the hinge. 



