OF SOUTHERN INDIA. SU 



elongated and narrow gills on each side, only free along the inferior edge ; the 

 foot is rather short, terminally expansible into a sub-oval disc, truncate or slightly 

 emarginated behind, pointed in front, with serrated edges, deeply suleated along the 

 middle sole, and capable of being folded together when retracted within the valves. 



The shells are elongated, small, or of moderate size, posteriorly always more or 

 less produced and often slightly gaping ; the hinge is composed of two comb-like 

 multidenticulated diverging marginal teeth ; ligament internal or external ; pallial 

 impression generally sinuated, rarely nearly entire ; internal layer of shell some- 

 times slightly nacreous, but more often only with a slight silky lustre. 



The jVuculanidje are characterized, as already stated, principally by the dis- 

 tinctly developed posterior siphons and the frequently sinuated pallial sinus, 

 the shells are produced posteriorly, while the reverse is the case in Nucula. 

 There is really a very great structural difference between Nuculana and Nucula, 

 and Deshayes' criticism of H. and A. Adams' and other authors' classifi- 

 cation is entirely uncalled for. It contributes nothing towards Deshayes' sug- 

 gested unity, but it speaks rather in favor of the separation of the two families. 

 No one will deny the relations of the two as to animals and shells, they are only 

 too well known, but when the differences of organisation extend to such cha- 

 racters as those upon which our whole systematic classification is based, it would 

 be the greatest mistake to ignore them in one and make use of them in another case. 



Eossil species of Nuculanid^ occur througli all the known sedimentary 

 formations, but they never are very numerous, and appear to increase very 

 gradually up to the present date. The palaeozoic species will barely amount to 

 15 or 20, those from the Trias about 25, from the Jurassic period about 40, 

 from the cretaceous 63, from the tertiary at least 80, and from the present seas 

 nearly 100. 



The geographical distribution is general ; the species of Nuculana live mostly 

 in tropical and sub-tropical, those of Toldia chiefly in the Northern, seas. 

 The usual depth at which they occur is between 2 and 4 fathoms, on slightly 

 gravelly, sandy, or muddy soil. During former geological periods the same general 

 distribution appears to be indicated, at least species belonging to the family have 

 been found almost in all countries and are aboixt equally scarce. 



According to the more or less distinct internal pearly structure and the 

 presence of an internal ligament on the one hand, the want of the former and 

 the replacement of the latter by an external ligament on the other, two sub- 

 families have been proposed by H. and A. Adams under the respective appellations 

 of NucuLANiNM aud MALLETiNJE. Thcse I shall here adopt, classifying the few 

 additional fossil genera according to their relations to recent forms. 



a. Sub-family,— NUCULANIN^. 



1. Cucullella, M'Coy, 1851, (Ann. Mag. Nat. Hist., 2nd ser., vii, p. 50). 

 Shell sub-rhomboidal or elongated, inequilateral, the beaks being sub-anterior, 

 internally with a long internal septum extending towards the anterior muscular 



4i 



