OP SOUTHEEX INDIA. 387 



In the arrangement of tlie genera, I shall here adopt three sub-divisions accord- 

 ing to the develojjment and location of the ligament : — 



a. AricrziXyE, with Megambonia, Ambonichia, Megaptera, Monopteria, Tteri- 

 nites, BJii/nchoptents, Posidouomya, Salobia, Fterinea, Pseiido-monoHs, Casianella, 

 JPteroperna, Aucella, Acicida. 



b. MELisiy^, with Actinodesma, Sornesia, Gercillea, Inoceramiis, Anopxsa, 

 BulvinUes, Ternostrea, LeproconcJia, Crenatula, Melina. 



c. vvLSELLis^, with ElUjiiius, Chalmasia, NaycuUna, Bimya, Vulsella, Malleus. 



a. Suh-family—AriCULINM 



Ligament attached to the entire external hinge margin, or placed in a 

 shallow groove near the beak and spreading over the hinge area as it extends 

 posteriorly. 



1. Megambonia, Hall, 1859, ( Pal., New York, iii, 273 ). Shell sub-ovoid, sub- 

 equivalve, moderately inflated, very inequilateral, with sub-anterior beaks ; anterior 

 muscular scar small, distinct, marginal, posterior larger, indistinct; hino-e line 

 anteriorly crenulated, posteriorly somewhat expanded, more or less compressed, but 

 not usually distinctly winged ; sm-face marked with lamellar striee of growth and 

 sometimes also wdth radiating lines; type, M. sub-orbicularis, Hall. 



The shells of this palseozoic genus strongly resemble ovoid forms of Modiolopsis, 

 ■with which probably some of the species are generally identical. Hall says 

 that the complete structure of the hinge line is unknown ; it is, therefore, impos- 

 sible to give a good characteristic of the genvis. Meek (Am. Journ., Sc. Arts., 

 xxxvii, 215, ) observes that some of the species of Megambonia, as M. aviculoides 

 and lamellosa, may possibly belong to a genus allied to, if not identical with, 

 Fterinia, "though the typical species appear to belong to the Arcidm." 



2. Ambonichia, Hall, 1813, (Pal. New York, i, 1G3, and 1859, iii, 269 and 

 523). Ptoundly sub-quadrangular, equivalve, moderately inflated, with anterior 

 incurved beaks and a posteriorly extended straight hinge Hue, anterior side 

 straight, below the beaks slightly insinuated and gai)ing, posterior truncate ; hinge 

 anteriorly below the beaks with a few short oblique, and posteriorly towards the 

 termination of the hinge line also with a few sub-parallel or slightly divero-ino-, 

 rib-like teeth; posterior muscular scar large, sub-central, anterior very small or 

 nearly obsolete; type. Am. radiata, Hall, from silurian rocks. 



3. Megaptera, Meek and Worth., 1SG6, (Geol. and Pal. of Illinois, vol. iii, 

 337). Sub-trigonal, sub-equivalve, posteriorly with a very large pointed win"-, 

 obtusely convexly angular from the beak to the inferior narrow end, beaks anterior, 

 terminal, incurved, slightly projecting above the hinge line, anterior wing probably 

 obsolete ; hinge with a few small anterior teeth below the beak, as in Ambonichia : 

 posterior muscular scar large, sub-central, pallial line extending anteriorly up to 

 very near the beak ; type, M. Casei, M. and W., from lower silurian rocks of 

 Richmond, Indiana. 



5b 



