184 I Senate 



The relations of this shell are among the Arcadce, and approximate to 

 the NucuI(B in their general character, and to which genus they have usu- 

 ally 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 palaeozoic species referred to the Genus 

 Nucula belong to Tellinomija, except those of the Genus Nuculites of 

 Conrad, CucullcUa of M'Coy, which is distinguished by the presence 

 of a septum in the anterior part of each valve. The place of Tellinoniya 

 may be regarded as between Nucula and Nuculites. In external charac- 

 ters it may prove difficult to separate Tellinomya from Nuculites, but the 

 presence of the septum affords nearly the same degree of difference as that 

 between Cucdllella and Area. 



This species of the Genus Cucullella of M'Coy 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 reason, 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 Silu- 

 rian or perhaps the Devonian period, while the Tellinomycs occur among 

 the earliest forms of lamellibranchiate shells. 



For the purpose of coaiparison with Tellinomya, and as exhibi- 

 ting 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 JYuculites and Cucullella^ 

 from the descriptions respectively of Mr. Conrad and Professor 

 M'CoY : 



NUCULITES. 



Conrad, 1841, Geol. Eep. of N. York. p. 49. 



" Equivalved ; hinge with cardinal teeth as in JYucula, but appa- 

 rently uninterrupted beneath the apex ; an interior rib like 

 that of Solecurtus, but narrower, extends from the apex, either 



• direct or slightly oblique, towards the base, never passing much 

 beyond the middle of the valve." 



" These shells have much the exterior aspect of Nucula, but the deep 

 sinus in casts of some of the species, left by the interior rib, constitutes 

 about the same amount of difference between the two genera as between 



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



