INVERTEBRATE PALAEONTOLOGY. 1 73 



* 



1 Have been much nuzzled in regard to the proper disposition of the 

 little group of small, very thin species here placed under the new subgeneric 

 name Leptocardia. These shells are generally so smooth, and show so little 

 indications of the characteristic posterior radiating costse, that there would 

 often seem to be no traces of them, yet the posterior third of the free margins 

 is always distinctly crenate within; while in front of this they are entirely 

 smooth. In addition to this, traces of these costse are clearly seen on the 

 outer surface of some individuals of the same species. The curious character 

 of having two little sinuses in the pallial line (see fig. 8, e, pi. 29) would also 

 seem to favor the separation of this little group. Whether or not it should 

 also include the subsection (b) cannot be positively determined until more is 

 known in regard to the hinge and interior of that type. 



Most authors have generally regarded Protocardia (or Protocardium, as 

 the name is often written) as itself only forming a subgenus under Cardium, 

 and it must be confessed that it scarcely differs more widely from some of 

 the generally-admitted sections of that group, than the latter differ from the 

 typical forms of Cardium. On the other hand, however, it seems to have 

 even closer relations to Lcevicardium than to Cardium proper. Indeed, if the 

 recent species L. /yratum, Sowerby, and L. eolicum, Born,* can be properly 

 included, even in a subgenus, under Ltevicardium, there would almost seem 

 to be equally good reasons for placing Protocardia near it as another subgenus 

 of the same group. The chief objection to this latter conclusion, however, 

 is, that we cannot trace, or at least have not yet traced, the Protocardia group 

 continuously through the Tertiaries, by an unbroken series, into the existing 

 seas; though it is represented in the Eocene of this country and Europe by 

 the very closely-allied group Neniocardium.\ 



This genus, so far as yet known, appears to have been introduced during 

 the Liassic epoch. The species, however, were more abundant during the 

 Cretaceous period, at which time the genus probably attained its maximum 

 development. 



* Whether these two recent species can be properly retained in the genus Lamicardium, or should 

 be referred to the nioro ancient group Protocardia, they certainly form a very marked subgenus, differing 

 from Lcevicardium proper in having radiating posterior strise or cos'a? and oblique anterior markings; 

 while they also differ from Protocardia in having the sculpturing on the anterior half of the valves 

 oblique, and running out on the anterior margin, instead of being concentric, or parallel to the marks of 

 growth. Lyrocardium would be a good name for this section. 



t Protocardia lima, Conrad (Am. Jour. Conch., 1, 139), from the Eocene of Mississippi, appears to bo 

 a true jfTeniocardia. 



