28-i 



SHELLS AND SHELL-FISH. 



PART I. 



cardinal teeth in the left valve ; whereas, in the Unio- 

 -sisM, this valve has scarcely ever more than one. 

 When, however, both are present in the right valve, 

 the inner one is always under, and not on the same 

 line or on the side of the outer tooth. The most 

 aberrant species of Mysca have something of this ap- 

 pearance, not very apparent to young conchologists. 

 But there is another and a more easy discrimination. 

 Of thirteen species of Iridea now before us, each and all 

 have the bosses or umhones wrinkled transversely, as 

 in Hyria. We cannot, indeed, expect that every species 

 will be so marked, for then there would be an absolute 

 line of demarcation between Iridea and Mysca, which 

 is contrary to nature ; but the facts now stated will 

 enable any one to discriminate the two types. The 

 next sub-genus is Lamarck's Castalia, of which but 

 one species is generally known. Reasoning from ana- 

 logy, it follows Iridea ; but if so, the minor types of 

 connection are wanting. It is a cockle-shaped shell, 



nearly equilateral, and 

 59 bears a strong resem- 

 blance to JEglia cune- 

 ata. From this rare 

 and somewhat isolated 

 type the transition to 

 Hyria (fig. 5^.) is ren- 

 dered easy and natural 

 by means of Hyria cor- 

 rugata Lam. ; and Hy- 

 ria elongata indicates a passage to that elongated form 

 which has not yet been discovered, and which must 

 represent either Alasmodon or Iridina^ Such a shell 

 we remember to have once seen at King's Auction- 

 rooms, but by some accident neglected to take notes, or 

 to secure the specimen. In the sub-genus Na'ia, which 

 we arrange under Castalia, we have a form connecting 

 the C. cordata* with Iridea. AV^e have before us six 



* I use the prior specific name of Ilumplircy rather than Lamarck's 

 ambigua, which throws an unnecessary tioubt on its being a species. 



