262 ROYAL SOCIETY OF CANADA 
pair of minute (sub-cardinal ?) muscle scars lie about half way between 
the cardinal muscles and the large laterals ofthe central group. 
À pair of vascular trunks extend from inside the lateral scars, for- 
ward for some distance beyond the central scars, angling inward : from 
the inside of these trunks, about six branches extend toward the centre of 
the valve, and finer branches radiate from their outer edges. The edges 
of the valve and the growth ridges of its outer surface are marked by 
numerous crenulations or furrows directed outward. 
The ventral valve is more elongated than the dorsal, and in some ex- 
amples has a wide, faint depression along the centre of the deltidial area. 
A triangular area on each side of the valve extending two-fifths from the 
apex marks the position of the lateral muscles; at the back of the tri- 
angle is the scar of the umbo-lateral, and at the outer corner is a group of 
three small oval scars, constituting the main lateral group of the crescent. 
The rhombic pit marking the position of the central group of muscles has 
a crenulated band along its centre due to the central muscles, and on each 
side are the prints of the laterals. 
The crenulations of the margin of the shell, where the vascular lines 
terminate (marked on the outside of the shell by radiating striæ) are about 
as far apart as the striæ on the outer surface spoken of by Mr. Billings. 
There is a slender median ridge to the valve, and on each side of this a 
calcareous thickening of the valves marking the edges of the pseudo- 
platform. The crescent in this species is of the nature of a parietal band 
as it incloses the muscles and does not form a broad band in which the 
muscle-scars are inclosed as represented in the figures of Messrs. Davidson 
and King for Trimerella. 
Var. cunEATA, PI. I, Figs. 4c and d. 
Among the examples in the Museum at Ottawa is an example of a 
ventral valve which shows considerable difference in the form and the 
markings of the interior. It is a deeper shell, with more abrupt lateral 
slopes and is more triangular in outline at the front. It is a mould from 
which the shell has been mostly exfoliated. There is no trace of a deltidial 
groove or depression ; the main muscle-scar of the crescent is undivided, 
and the anterior muscle-scar of this group distinctly visible is small : the 
sides of the pseudo-platform arch outward, the large, central muscles have 
left double scars, and the median septum is more decidedly shown than in 
the typical form. A distinct parietal band has a position similar to that 
in Lingula, The example studied is a quarter shorter than full grown 
examples of the typical form. 
We cannot discover that there is any hollow platform in this species 
as there isin Trimerella, the central thickened part of the shell of the 
ventral valve corresponding to the platform in the ventral of that genus ; 
