INVEKTEBHATE PALEONTOLOGY. 167 



7. NEMOCARDIUM, Meek. 



Shell closely resembling the typical forms »of Protocardia, but 

 thinner, with two-thirds to three-fourths of surface in froni of the 

 stronger posterior, usually echinate, radiating costa?, occupied by fine, 

 crowded, radiating stria*, and the free margins crenate within all 

 around; cardinal and lateral teeth generally rather slender; pallial 

 line faintly sinuous, irregularly serrated, or nearly simple behind. — 

 Cardium semi-asperum, 1 >eshayes. 

 The foregoing is essentially the arrangement of H. and A. Adams, 

 though I have added the group Criocardium of Conrad, and IVemocardium, 

 and left out Serripes. The first of these is an extinct section, presenting the 

 singular* character of having spines or nodes arranged in the furrows between 

 the costae, instead of along the crests of the same ; while the Eocene section 

 Nemocai-dium forms a curious transition from the Cardium to the Protocardia 

 group.* In regard to Serripes, it may be remarked that, although perhaps 

 presenting no fundamental differences from the genus ('indium, it neverthe- 

 less seems, on account of its compressed transverse form, smooth surface, 

 and sometimes obsolescent cardinal teeth, to have about as strong claims to 

 rank as a distinct genus as LcBvicardium, to which it is in several respects 

 more nearly allied than to Cardium proper. 



I have also viewed Cardium aculeatum as the typical form of the genus, 

 instead of C. costatum (cited by H. and A. Adams and others as the typical 

 species), thus making Cardium proper exactly identical with Acanthocardium, 

 Gray. This seems to me to be necessary, according to the (in most cases 

 good) rules adopted by the British and American associations; though the 

 rule of taking the first species mentioned by an author in founding a yenus 

 (followed by many) would require C. costatum to be taken as the type, that 

 being the first species in Linnaeus' list. But, as he also included species 

 belonging to La 'vi cardium, Hemicardium, &c, the most generally adopted 

 rule requires that we should look to some of the systematists who first 



" The Nemocardium section stands, as it were, so exactly intermediate in its characters between the 

 Cardium and Protocardia groups, thai it might, with almost equal propriety, 1"- ranged as a section of 

 either. Deshayes even places the nine Paris-basin species of this type under Protocardia, which he views 

 as a section of Cardium propel-. It differs from Protocardia, however, in having tine, radiating eostee, 

 instead of concentric markings on the anterior two thirds of the valves, and the free margins crenate 

 within all around, instead of only posteriorly ; aiso in being thinner, and having a weaker hinge than is 

 usual in the typical species of that group, as well as in the irregularly-serrated appearance ot the pallial 

 line posteriorly in some of the species. 



