ANIMALS. 91 
from its projecting so far behind the hinge-margin ; and from the vertical position of the 
face on which the muscle it supported has been implanted, it would appear that the 
opposite end of this (cardinal) muscle has not been attached to any part of the 
umbonal cavity, in front of a transverse line drawn exactly vertical to the structure in 
question. Nothing can be more satisfactorily clear than the origin of the reniform 
impressions (f), as mdicated in the figure,—an origin which, when considered in 
connexion with what is displayed in the fossils represented elsewhere (vide Pl. XIX, 
fig. 37; Pl. XX, fig. 7/7), may now be considered as established beyond a doubt. 
What are called the anterior divisions of the valvular muscular impressions (a), are also 
finely displayed on the gutta-percha casts; but it is to be regretted, owing to the 
small size of the figure, that certain characters which distinguish them from the 
posterior divisions (c), cannot be so faithfully represented as in the original: they are 
more elevated than the latter, and less complexly dendritic. The dotted impressions (e) 
are not often visible ; but having seen the same on various casts, I feel little doubt of 
their being due to the attachment of some muscles,—the inferior pedicle muscles very 
probably. The spine-like tubercles crowding the inner surface of the valve, are often 
very finely preserved: they have evidently increased with age, as I have seen large 
specimens in which those situated im front were nearly thrice the size of their 
analogues on the specimen figured. 
The structure of the shell of Productus horridus is represented in fig. 13, Pl. XI: at 
a a magnified view is given of the thin superficial lamina, with its striz of growth ; 
and at 4 the substance of the valve (the large one), which consists of closely- 
packed fibres (? capillary tubuli), running nearly parallel to the outer layer. 
Hitherto no one has identified this species with any forms occurring in other 
formations than the Permian; but 1 am strongly disposed to think that, before long, 
its chrono-geological range will be found to be more extended. I am inclined to this 
view, from having seen a specimen in the London Geological Society’s Collection, 
labelled “‘ Producta Leuchtenbergiana, from Lough Macnean, Fermanagh,’ bearing a 
striking resemblance to this species. It is only an examination of a number of 
specimens of the latter shell that would enable me to pronounce a positive opinion as 
to its specific identity with the Permian species. 
Productus horridus is a widely-distributed fossil. As British, it was first described 
(1823) by Mr. J. Sowerby, who states it to be “from the seventh bed of Mr. White 
Watson’s first Limestone, probably Magnesian, as it is above the coal series” in Derby- 
shire, “where it is not very rare.” (Min. Conch., vol. iv, p. 17.) It also occurs very 
abundantly in the Shell-limestone at Humbleton Quarry ; not quite as much so, in the 
corresponding rock, at Tunstall Hill and Dalton-le-Dale. A few specimens occurred 
to me in the Breccia at Tynemouth Cliff: it is also found in the compact Limestone at 
Midderidge, Garmundsway, Millfield Quarry (Sunderland), and Whitley. Professor 
Sedgwick, who notices most of the foregoing localities, also states it to occur 
