Mr. il. Etheridge on Carboniferous Mollusca. 33 



f. 13, b, d), appears to possess the hinge-structure of Entolium, 

 with the radiating internal costee of Amussmm. 



2. On the Hinge-structure of Nucula gibbosa, Flem.^ and 

 Nuculana attenuata, Flem. 



These shells were placed by the late Mr. J. W. Salter in hia 

 genus Ctenodonta^ , in which he has been followed by several 

 other palseontologists, including, at one time, myself. A fur- 

 ther examination of the subject, however, convinced me that 

 this step had probably been taken by Mr. Salter without due 

 consideration. Ctenodonta appears to be synonymous with 

 Tellinomya^ Hall, the latter having precedence in date, al- 

 though Salter's definition of his genus was perhaps more com- 

 prehensive than that of Hall as originally given. Tellinomya 

 is an undoubtedly good genus, and will, I anticipate, be found 

 to have attained its greatest development in the Silurian rocks, 

 although I have no doubt it extends into the Carboniferous. 



The essential character of Tellinomya (or Ctenodonta), as 

 distinguishing it from Nucula or Nuculana, is the entire ab- 

 sence of an internal cartilage-pit and the substitution for it of 

 an external ligament ; whereas in the two latter genera the 

 cartilage is deposited in a well-marked pit beneath the um- 

 bones, and between the two ranges of teeth, anterior and 

 posterior, in both valves, and there is no external ligament. 

 One or other of these essential characters must therefore be 

 shown to exist in the above species before their respective 

 affinity or want of affinity with Tellinomya [= Ctenodonta) can 

 be shown. Prof. M'Coy, many years ago, noticed the presence 

 of a cartilage-pit in Nucula gibbosa, Flem.f He says, " I 

 have clearly ascertained the presence of the angulated line of 

 hinge-teeth and the intermediate cartilage-pit of Nucula^ I 

 have personally seen one or two good and clean interiors of 

 this shell, and am quite able to corroborate Prof. McCoy's 

 statement of the existence of a cartilage-pit in N. gibbosa, 

 and, in consequence, the reference of the species to the genus 

 Nucida, made by Fleming, M'Coy, and other palaeontologists. 

 In N. gibbosa the hinge-line is curved, the anterior range 

 of teeth being a little less than half the length of the 

 posterior. In the best-preserved specimen I have seen (a left 

 valve, figs. 8 & 9) there are twenty teeth on the posterior side of 

 the cartilage-pit, and five or six on the anterior, in both cases 

 the denticles enlarging as the cartilage-pit is receded from. In 

 form the denticles are roughly triangular and projecting, with 



• Iron Ores Gt. Brit. pt. 3, p. 221. 

 t Brit. Pal. Foss. p. 512, 



Ann. rfe Mag. N. Hist. Ser. 5. Vol. ii. 3 



