392 CllETACEOUS PELECYPODA 



indicated by a posterior insinuation, as in ^. praetexta or tegulata, and those species 

 directly connect Avicula and Meleagrina, which certainly cannot be separated into 

 two equally valid genera. Young shells of all the three divisions are almost undis- 

 tini-uishable, having the posterior wing very small, or not all developed. Species of 

 Amcula occur in all sedimentary formations from the oldest up to the most recent. 

 Many of those from palasozoic rocks are, however, said to belong rather to Fterinea 

 and allied genera than to Avicula. 



o"- 



b. Suh-family,—MELimNyli. 

 Cartilage situated in a number of transverse, marginal, grooves of the 

 hinge line. 



16. Actinodesma, Sandberger, 1856, (Rhein. Schichtensyst. in Nassau, 

 p. 282). Slightly obliquely and broadly oval, moderately convex, with a long 

 straight hinge line, produced on either side into a narrow wing ; hinge with a 

 number of ribs inclined towards the horizontal hinge line on either side of the 

 central area on which they are absent ; these ribs are separated by grooves in which 

 the ligament is said to be lodged, being almost quite internal; type, A. mallei forme, 

 Sandb., from the Devonian; only one species known. 



I do not think that it has been sufficiently established that the grooves alluded 

 to are really ligament- or cartilage-grooves. They rather appear to me to be iden- 

 tical with similar hinge ribs of Fterinea and Gercillea, and the ligament may have 

 been external and marginal, attached to the thickened margin of the shell which 

 slopes internally, as is, for instance, the case in most species of Avicula and particu- 

 larly in the Ileleagrimi group. If this were the case, the genus had to be trans- 

 ferred to the AVICULIN^. 



17. Hornesia, Laube, 1866, (Denksch. Akad., Wien., xxv, pt. ii, p. 52). 

 Obliquely elongated, solid, inequivalve, left valve inflated, with incurved beak, right 

 more or less flattened, hinge line straight, with a short narrow somewhat contorted 

 anterior and a long posterior wing, not separated from the body of the shell, 

 except by a shallow marginal insinuation ; ligament situated in several pits 

 externally on the hinge line, one pit being below the beak and reaching rather 

 internally, one is on the anterior and the remainder on the posterior side ; hinge in 

 the left valve consisting of a strong oblique tooth under the beak, separated by a 

 pit from a smaller anterior cardinal; in the right valve there is only one strong 

 tooth, besides that there are generally numerous crenulations at the margin of the 

 hinge line in both valves, and one or two oblique sub-marginal ribs posteriorly ; 

 muscular scars two, deep, close together, not far from the umbones ; type, M. Joannis- 

 Austri(C, Klipst., sp., from the Trias. Several of the triassic species of Gervillea 

 most likely are referable to this genus, fvide Credner in Bronn's Jahrb. fUr Min,, 

 &c., 1851, p. 611, pi. vi). 



18. Gercillca, Defr., 1820. Very obliquely elongated, generally narrow, sub- 

 oquivalve ; hinge line straight, with small wings, the anterior being much shorter 

 than the posterior, and neither of them is distinctly separated from the body of 



