PART 3.] 
Theobald : On the shells of the ossiferous beds. 
55 
Three species of Unio occur in the collection, U. marginalis, Lam.; U. cjerulees, Lea ; 
and U. corrugates, Mull., which last embraces four distinct races,usually classed as species 
by most authors, but which, after some study of the Indian forms of the group, I incline 
rather to treat as local and permanent races, thereby reducing within manageable and natural 
limits the crowd of shadowy species, with which the literature of the grdup is burdened. 
IT. marginalis, Lam. 
This species is not uncommon, but is not so finely developed in proportion, in the ossi¬ 
ferous gravels, as the others, neither does it seem to occur quite so well preserved, nor to 
obtain the same weight of valves as in the other species, in which respect it simply agrees 
with the same species now living, which never displays any considerable thickening or 
calcification, under any conditions, however favorable. A perfect example, not fully grown, 
measures— 
Breadth ... ... ... 57 1 
* Length ... ... ... 29 V Mills. 
Thickness ... ... ... 19 j 
Allowing for slightly broken edges in the fossil, these proportions closely accord with the 
living shell which I give from Manbhoom— 
Breadth ... ... ... 61 j 
Length ... ... ... 31 > Mills. 
Thickness ... ... ... 18 ) 
A second specimen from these beds, which may be considered fully adult, measures 93 
mills by 43. 
U. CiERULEES, Lea. 
This species attains to a superb development in the ossiferous gravels, and merits nomi¬ 
nal recognition, since it does not quite correspond with any variety hitherto separated. It 
may stand as var. Namadicus, Theob. 
Two perfect examples measure respectively— 
a. 
b. 
Breadth 
56 
46 
Length 
31 
24 
Thickness 
23 
17 
A precisely similar form is now living in the Narbada, and differs less, from the type of 
the species, than some other races in other parts of India do. It agrees generally in the 
form of the teeth, in shape, color, and sculpture, save that each character is heightened in the 
Narbada form. The lateral teeth often display a carneous tinge, and the sculpture of the 
valves is not only stronger than in the type, but covers a far greater area, both on the valves 
and their posterior slope. A very similar form, though departing more from the tyj>e as 
regards shape, inhabits the Kistna valley, where it attains a breadth of 60 mills—(the typo 
measuring only 43). 
U. CORRUGATES, Mull. 
It is a groat pity that the type of that species of Unio, which seems to unite the greatest 
number of races in India, should be so ill characterised, difficult of identification, and appa¬ 
rently, with, a good series under view, so aberrant from the more strongly marked forms, 
which strict zoological argument requires should be united to it. On this subject I would 
Length is measured at right angles to a line tangential to the ventral margin. 
