338 CEETACEOUS PELECYPODA 



Conrad (Am. Journ. Conch., Ill, 1867, p. 9), quotes T. Maconensis, Con., 

 fi'om North American cretaceous beds as the type of Trigonoarca, (see Journ. Phil. 

 Acad., 2nd ser., iv, pi. 47, fig. 20). Area Ligeriensis, d'Orb., would be a still 

 better example. The cretaceous species are very numerous, and many show con- 

 siderable variations in the hinge-teeth. Some, like A. Ilarreana, d'Orb., appear to 

 indicate a direct transition to Qacullmi. It is possible that the so-called CucuUcea 

 polijgli/pha, Laube, from the triassic beds of St. Cassian also belongs to this genus. 

 It only differs by having the surface costated (see Denk. Akad., Wien, xxv, pt. ii, 

 p. 62) ; it may, however, also be referable to Noetia. 



10. Latiarca was described by Conrad in Proc. Phil. Acad., 1862, p. 289. 

 Unfortunately the first portion of this publication for that year appears to have 

 gone astray, for in our own library, as well as in the Asiatic Society's, the volume 

 for 1862 remains incomplete up to date. The reference to the genus in Am. 

 Journ. Conch., I, p. 11, leaves me in doubt about the characteristic which Conrad 

 assigns to it. He quotes from lower eocene ( ? cretaceous) beds three species, 

 L. gigantea, ononchella, and transversa. The description and figure of the first, in 

 Journ. Ac. Nat. Sc, 1st ser., vi, p. 227, pi. 10, fig. 4, are so incomplete that no 

 certain opinion can be based upon them. Conrad says, the anterior side is flat- 

 tened and produced, the posterior very short, but the figure appears to indicate the 

 type of a shell which resembles a short Cucullcea, or perhaps a Noetia. The refer- 

 ence to the two other species is also not accessible to me at present. 



11. Noetia, Gray, 1842. This genus, with N. reversa. Gray, as type, seems to 

 represent in the recent fauna the group of fossil shells to which I have just pointed 

 as Latiarca, and possibly both may be the same, but I have none of the recent 

 species to compare. Their form is sub-trigonal, ventricose, sub-inequilateral, radi- 

 ately ribbed, and covered with a dark foliaceous epidermis ; the ligamental hinge 

 area is narrow, but distinct, the hinge line slightly arched, with transverse oblique 

 teeth placed so as to converge towards the centre of the shell ; these teeth are 

 more numerous on the anterior than on the posterior portion of each valve. 



12. Lunarca, Gray, 1842. Shell globose, cordiform, sub-equivalve, with the 

 beaks incurved and close together, the ligamental hinge area being very narrow 

 and almost absent, surface radiately ribbed ; hinge line angular, posterior part 

 nearly straight, long, and provided with numerous short transverse teeth ; anterior 

 portion shorter, bent downwards, forming in the left valve an entire, prominent 

 sub-lunular tooth, which fits into a corresjionding pit of the right valve ; type, 

 L. costata. Gray, from American seas. This is a remarkably distinct type of 

 Arcid^. I am not acquainted with any recent or fossil species exactly corres- 

 ponding to it, but the palgeozoic Cardiola shows externally a great resemblance to it. 



13. Isoarca, Munst., 1842, (Beitrsege z. Petrefacten-Kunde, &c., 6tes Heft, 

 1843, p. 81). Shell ovate, elongated, inequilateral, ventricose, beaks tumescent, 

 sub-anterior, strongly incurved ; ligamental hinge area wanting, represented only 

 by a narrow groove extending from the beaks posteriorly : hinge line long, nearly 

 straight, or slightly arched, provided in its entire length with numerous, short, 



