254 Mr. T. R. Jones on North American 



cially as it appears to me that we have here an interesting zoo- 

 logical link between the fauna3 of different regions. 



A specimen of "Medina Sandstone/' containing Lingula 

 cuneata, from Rochester Creek, Niagara, in the Geological 

 Society's Museum, contains several coarse sandy casts of a Le- 

 perditia {suh^enns Isochilina) , occasionally J inch long and ^^ inch 

 broad, resembling the specimens from Russia figured in Annals 

 Nat. Hist. 2 ser. vol. xvii. pi. 7. figs. 11 & 12, which were re- 

 ferred with considerable doubt* to L. marginata of Keyserling, 

 and are most probably of the same species as those figured by 

 Eichwaldf under the name of " Cypridina Balthica.'' 



It is easy to find on the piece of sandstone before me imper- 

 fect or partially imbedded specimens more or less resembling 

 the figures given by Prof. Hall ; and, should they prove to be 

 the same as his, I shall be inclined to apply his name of " cylin- 

 drica " to " the smaller form of L. marginata % " ; and, as I have 

 already suggested its independence of the large L. marginata, 

 being itself probably an adult form, and that it would belong, in 

 that case, to a subgenus of Leper ditia, I feel little hesitation in 

 grouping it with the Isochilince described above § (p. 248). 



(Beechey Island : additional.) 



5. Cytheropsis concinnat PL IX. fig. 3. 



Length -^^y breadth -^j inch. 



Oblong-ovate, somewhat Leperditia-shaped ; ends unequal, 

 ventral edge well rounded ; surface smooth. 



This very much resembles C. concinna (above, p. 249). 

 Several specimens occur in the limestone from Beechey 

 Island. 



The following Table exhibits a general view of the genera and 

 their species as yet known in Arctic America, Canada, and the 

 States. 



* Op. cit. p. 91. There is an important error to be corrected in the 

 page here referred to : at line 7 from bottom /or fig. 6 read figs. 7 & 8. 



t Bullet. Imp. Soc. Moscou, 1854, no. 1. p. 99. pi. 2. figs. 7 & 8. " C. 

 Baltica" and " C. minuta'* are also quoted by Capt. P. Jeremejew, as 

 occurring at Iswos, &c. Verhandl. R.-K. Min. Ges. 1856, p. 83. 



X Op. cit. p. 94. 



§ In connexion with the Russian specimens above alluded to, M. Eich- 

 wald mentions a form having a row of punctiform pits on the border of the 

 valve (Annals, loc. cit. p. 93). This is an important feature also in the 

 IsochilincB above described from Canada (see p. 248). 



