PRODUCTUS. 137 



the hinge-line, a prominent bilobed or trilobed projection or cardinal process (j), 1 its 

 upper surface is often striated, and affords attachment to the cardinal or divaricator 

 muscles (r). Under this a narrow longitudinal ridge generally extends to about half (or 

 more) of the length of the valve, and on either side are seen the ramified or dendritic 

 impressions, which we consider to be attributable to the adductor or posterior and anterior 

 occlusor muscle (a), bnt which are often situated so close to each other, on either side of 

 the mesial ridge, as to render the quadruple attachment not so distinct as could be desired ; 

 they are, however, well defined in P. longrispinus, and in some other species. Outside, 

 and in front of these, are the two " reniform impressions" (x). Their surface is generally 

 smooth, and bordered by ridges, which after dividing the occlusor muscles proceed in an 

 outward, oblique, or almost horizontal direction ; then turning abruptly backwards, they 

 terminate at a short distance from their origin. There exists also, in many species, but 

 not in all, two prominences (w), one on each side of the median ridge, and close to the 

 base of the muscular scars ; they are very apparent in Producius, but not observable in 

 Chonetes. 



The internal surface of the valves (in all the family) is covered with innumerable 

 granulations, of which some have been thought by Mr. Hancock to have been " probably 

 produced by the muscular bands, which retracted the margin of the mantle." I will 

 now describe the internal appearances observable on the concave surface of the larger or 

 ventral valve. A narrow mesial ridge, originating under the extremity of the beak, 

 separates two large, elongated, ramified or dendritic impressions (a), which have in all 

 probability been produced by the adductor or occlusor muscle, although otherwise referred 

 by some Palaeontologists. In advance of the larger scars, we sometimes perceive smaller 

 impressions closely connected with the larger ones above described (e), and which were 

 in all probability produced by a portion of the occlusors themselves. On either side of the 

 adductor or immediately under there exist two deep, longitudinally striated, sub-quadrate 

 impressions, which are due to the cardinal or divaricator muscles (r). I have in vain, 

 hitherto, sought for impressions referable to the adjuster muscles ; but as no peduncular 

 aperture existed, such muscles may be supposed not to have existed : however, as the 

 valves in the species hitherto known of Productus possessed no teeth or sockets, and 

 therefore are not strongly articulated, as in the Terebratidida, it is not impossible that the 

 adjusters may have been so arranged as to keep the valves adjusted to each other, and 

 that they thus acted as a substitute for a hinge. 



The only point remaining to be here noticed in connection with the interior of this 

 valve are the deep concave, often sub-spinal, depressions (l), visible in some species, such as 

 P. giganteus, P. humerosus, &c, and which were hollows no doubt occupied by the spiral 

 arms, for it would seem impossible to conjecture otherwise, how they had originated. 

 Similar hollows could not of course be expected to be present in those species in which 



1 Spe PI. XXXVII, figs. 1 -4. 



18 



