CANADIAN FOSSILS. 35 



subspiral, and in the typical species (I. cordiformis, Schiibler, and 

 I. subspirata, Minister,) quite toward one side. It may be doubtful 

 if such species as I. lineata of Minister and Nucula elliptica of Goldfuss> 

 which have subcentral beaks, do not belong to Ctenodonta ; they are 

 smooth, or with a concentric lineation only, while Isoarca is often 

 cancellate. 



Solenella, Sowerby, has an external ligament like Ctenodonta, but a 

 notched pallial line. 



The genus has been referred to in the third edition of " Siluria," 

 p. 213, 859. It will include, as I believe, all the Silurian Nuculae, 

 and a considerable number of the other Palaeozoic species. It pro- 

 bably extends, as above stated, into the Trias. 



C. NASUTA. 



Plate VIII. Figures 1,2. 



C. biuncialis at ultra, transverso-ovata, loevis, nisi antice rugis concentricis ; 

 latere antico rotundato, postico subcontracto elongato, hand carinato ; 

 umbonibus depressis. 



Synonym. — Tellinomya nastita, Hall, Palaeontology New York, vol. i., 

 plate 34, figure 3. Ctenodonta Logani, Salter, in British Asso- 

 ciation Reports, 1851. Trans. Sect., page 63. Isoarca Logani, 

 Woodward, Manual Shells, page 269. Tellinomya nasuta, Hall, 

 Report of the Regents of the University, Albany, 1857, p. 143, 

 fig. 1-3. 



A beautiful species, not unlike in shape to some species of Anatina 

 or Thracia, and, from the subcentral position of the beaks, very 

 unlike the usual appearance of Nucula. 



It is the largest known species of the genus, full two and a half 

 inches wide, by one inch and a quarter long, measuring from the 

 slightly prominent beaks, which are much nearer the anterior than 

 the siphonal end. The depth of the valves, united, comprises three 

 fourths of an inch. 



The general contour is transversely ovate. The anterior side 

 perfectly rounded and marked with concentric rugae, which are 

 strongest on the upper portion. The posterior side narrows consi- 

 derably, and is a little contracted at the posterior third; its surface 

 is smooth and gradually less convex towards the subtruncated end ; 



