428 Mr. R. Etheridge on Carboniferous Lamellibranchiata. 
? Modiola, sp., Rhind, Age of the Earth, 1858, p. 167, t. 2. f. g (without 
description). 
Mytilus (Myalina) crassa, King, Mon, Perm. Foss. Engl. 1850, p. 159 
(without description). 
Mytilus crassus, Morris, Cat. Brit. Foss. 1854, p. 214 (without de- 
scription). 
Myalina crassa, Huxley & Etheridge, Cat. Foss. Mus. Pract. Geol. 1865, 
p- 110 (without description). 
Myalina crassa, Armstrong & Young, Cat. Carb. Foss. W. Scotland, 
Trans. Geol. Soc. Glasgow, 1871, iii. Supp. p. 52 (without description). 
Spec. char. Shell elongate, obliquely subtrigonal, very thick, 
with strongly marked umbonal ridges, inequivalve, large 
when full-grown. Anterior end forming a small lobe in 
front of the beaks and umbonal ridges; its margin sinuous, 
and a little concave in outline. Posterior end compressed 
in comparison with the anterior; its margin slightly sig- 
moidal in outline. Hinge-line straight, nearly equal to 
the length of the shell. Beaks not quite terminal, small 
and pointed, a little incurved at their apices. Umbonal 
ridges prominent, extending from the beaks backwards and 
downwards to opposite the anterior part of the ventral margin, 
where they become lost in the general surface of the shell ; 
they are most prominent immediately below the umbonal 
region, and give to the valves a carinate appearance. Hinge- 
plate broad, with the inner margin a little thickened, and 
longitudinally marked by a very large number of cartilage- 
furrows. Byssal sinus in the right valve more developed m 
some individuals than in others. Anterior adductor impres- 
sion double, pit-like, and deep, placed within the umbonal 
cavity. Posterior adductor impression large and transversely 
elongated. Pallial line well marked in old individuals by a 
series of pits along its course ; remote from the margin of the 
valves. Shell-substance very thick, especially in the umbonal 
region and cardinal area. Surface covered with an epidermal 
investment, ornamented by close fine strie. 
Obs. M. crassa was in the first instance referred to by the 
late Rev. Prof. Fleming as a Modiola in his paper on the 
Testaceous Annelides, as quoted above, but was afterwards 
briefly described by him as a fossil species of the genus 
Mytilus. With this short description, and a figure of a 
Modiola in Rhind’s ‘ Age of the Earth,’ from the water of 
Leith, near Woodhall (where it occurs in some abundance), 
which I believe to be the same shell, the early history of J. 
crassa is completed, so far as the literature of the subject is 
known to me. The greatest number of cartilage-furrows I 
have counted on the hinge-plate of any one specimen is 
twenty-four. These grooves are apparently continuous round 
