67 
valve. The ventral valve is slightly but pretty regularly arcliecl in tlio 
median portion and depressed towards the lateral extremities. The beak, 
little developed and slightly curved, does not project beyond the hinge line 
which is provided on either side of the beak with four or live rather short and 
slightly curved tubes. The dorsal valve is faintly concave and follows at a 
slight interval the contour of the opj)osite valve. The exterior surface of 
the two valves is ornamented with fine radiating ribs which increase slightly 
in number towards the margin either by a simple forking or by intercalation, 
so as to preserve almost the same tliickness, at any rate over half their extent; 
these ribs are not angular, but very slightly granular ; on well preserved 
specimens they are sixty to seventy in number. 
The ventral valve is provided, on the interior, witli a median septum 
faintly marked, but extending from the beak right to the lower third of tlie 
shell ; on its flank (sides) may be seen the impressions, often very indistinct, 
of the adductor and cardinal muscles ; almost all the inner surface is covered 
with small close-set prickles, arranged in radiating rows, corres^Doiiding 
exactly with the direction of the external ribs. I have not had an oppor- 
tunity to examine the internal structure of the dorsal valve. 
Dimensions . — The width of the largest specimens I have before me 
does not exceed twenty millimetres ; their length is twelve, and thickness one 
and a half millimetres. 
Relations and Differences . — There is no species of this genus about the 
limits and characters of which there is less agreement than this. My learned 
friend, Mr. Davidson, is convinced that the Carboniferous species he has 
deseribed under the name of Chonetes Sardrensis, is identical with that which 
J. Phillips descriljed under the same name, and which comes from the Upper 
Devonian of the neighbourhood of Barnstaple, and this opinion is so positive 
that he thinks there can be no doubt on the subject.^ In spite of the great 
admiration I have for his works, and the great esteem in which I hold him, I 
cannot bring myself to agree with his opinion, I have a strong conviction that 
the two Chonetes I have just referred to are different species, and this opinion 
is based on the study of a great number of specimens coming from different 
localities, and even from different countries. I have, moreover, obtained 
possession of, for comparison, specimens from the same locality as J. Phillips 
obtained those he described and figured, and for this I am indebted to the 
* Mon. Brit. Dev. Bracli., p. 95. 
L 
