1853.] DAVIDSON ON THE CHONETES COMOIDES. 203 



possessing a convex and concave valve, finely striated, with a flat 

 parallel space between the beaks. Better illustrations were, however, 

 subsequently published by Count Keyserling, and reproduced by 

 M. de Koninck : therein the characters of the double area, cardinal 

 process, and fissure are faithfully delineated ; but the recent discovery 

 of several more perfect examples has made it desirable to publish a 

 few observations and illustrations additional to those we already 

 possess. 



The Chonetes comoides may be described as a large semicircular, 

 inequivalve shell, with a straight hinge-line, generally (but not 

 always) as long as the greatest width of the shell, and articulated by 

 means of teeth and sockets. The ventral or dental valve is exte- 

 riorly more or less convex, with ear-shaped expansions. The dorsal 

 or socket valve concave, following the curves of the other. The beak 

 of the ventral valve is variably developed in different examples, — a 

 circurfistance due to the lesser or greater convexity of the shell : thus 

 in specimens with a moderately convex valve the beak is rarely pro- 

 duced or visible above the cardinal edge, while in more convex indi- 

 viduals (as in the typical examples figured in the ' Min. Con.') the 

 beak is more or less inflated, incurved, and produced beyond the 

 level of the cardinal edge. Each valve is provided with a subparallel 

 area, rather narrower in the smaller valve. In the middle of this 

 last exists a produced trilobed cardinal process {not tooth), which 

 enters and almost fills a corresponding triangular cavity or fissure in 

 the ventral area'^~ . The external surface of the shell is finely stri- 

 ated. Of the interior of the dorsal and characteristic valve unfor- 

 tunately no example has been hitherto obtained, and all we may 

 observe is, that it was provided with a well-developed cardinal pro- 

 cess, and sockets for the reception and articulation of the strong teeth 

 of the ventral valve ; but I have little doubt that the interior of this 

 valve was not only provided with the muscular impressions of Chonetes, 

 but likewise with the reniform impressions common to all the genera 

 or subgenera composing the family PRODUCTiD.Ef ; but of the inte- 



* This portion of the area in Mr. Sowerby's specimen was concealed by matrix. 

 The cardinal process does not concur in the articulation of the valves, which last 

 function is entirely performed by the means of special teeth and sockets ; the fissure 

 in this and other similar cases serving simply to afford space for the cardinal pro- 

 cess. This fact was clearly demonstrated by Prof. King and others ; but often very 

 erroneously termed a tooth by various authors. To this process were, no doubt, 

 fixed powerful cardinal muscles ; a point anatomically proved by the examination 

 of the same process in genera and species still alive (see General Introduction to 

 niy work ou British Fossil Brachiopoda, PaljEontograpli. Soc. 1853). 



■f Prof. M'Coy considers Chonetes to be simply a subgenus o( Leptcena, removing 

 it, as well as Strnphalosia (King) and Aidosteges (Helmersen), from tlie family 

 PiioDUCTiD^ (British Pal. Foss. Cambridge Museum, p. 3S7, 1852), and placing 

 them among his Lepttsnldce or OrthisiDjE ; but that is far from being the ease, 

 as I have endeavoured to demonstrate in p. 112 of my ' General Introduction,' 

 when showing that the reniform impressions (supposed to be vascular) are the same 

 both in Choiietes and Productus (see the annexed Plate VIII. figs. 1 1 and 13), while 

 a completely different arrangement in the vascular system seems to have prevailed 

 among the Strophomenid^, of which Leptcena constitutes simply a section. 

 VOL. X. — PART I. P 



