OSERVATIONS. 



This pretty species was presented to me by Nuttall as 

 having been taken from the stomach of a fish at Nahant, 

 Massachusetts. 



It resembles the N. Jlumatili,% SchriJter, and the N. ros- 

 trata. Montague, but the rostrum is not truncated and tlie 

 summit is more central than in those species. It is still 

 more closely related to N. Isevis^ Nob., but it is proportion- 

 ably a little longer, and in that species the posterior series 

 of teeth is a little arquated and the compressed anterior 

 hinge margin extends quite to the tip. I must say how- 

 ever that I have seen but a single specimen, and but a sin- 

 gle valve of the present shell. The N. arctica, Gray, which 

 this might be supposed to resemble, is said to be " posticc 

 brcvi, oblique truncato." 



REFERENCE TO THE PLATE. 



Upper figure — exterior of a valve. 

 Middle figure — back of a valve. 

 Lower figure — inside of a valve. 

 PLATE XII. 



For comparison I add, from the Journal of the Academy 

 of Natural Sciences, the following description and figure 

 of N. laevis, and also those of a smaller and quite distinct 

 fossil species. 



