OP SOUTHERN INDIA. 387 



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

 ing to the development and location of the ligament : — 



a, AvicuLiNjE, with Megambonia, AmhonicMa, Megaptera, Monopteria, Fieri- 

 nites, Bhyncliopterus, Fosidonomya, JBLaloUa, Fterinea, Fseudo-monotis, Casianella, 

 Fteroperna, Aucella, Avicula. 



b, MULiNmm, with Actinodesma, Hornesia, Gervillea^ Inoceramtis, Anopcea, 

 Fulvinites, Fernostrea, Leproconcha, Crenatula, Melina, 



c, VUL8ELLINM, with Eligmus, Chalmasia, Nayadina, Dimya, Vulsella, Malleus. 



a. Sul-family,—A VICULINM, 



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. Megamhonia, 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; hinge line 

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

 not usually distinctly winged ; surface marked wdth lamellar stri^ of growth and 

 sometimes also with radiating lines; type, M. sub -orbicular is, Hall. 



The shells of this palaeozoic 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 genus. 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 Arcid.^J' 



2. Ambonichia, Hall, 1813, (Pal. New York, i, 163, and 1859, iii, 269 and 

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

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

 straight, below the beaks slightly insinuated and gaping, 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 diverging, 

 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., 1866, (Geol. and Pal. of Illinois, vol. iii, 

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

 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 Anibonichia : 

 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 



