ANIMALS. 163 



differ remarkably in the position of their cartilage fulcra, which in the former are 

 lamelliform, elongated in the direction of the cardinal line, and, as in Lutraria elliptica, 

 directed downwards within the dorsal cavity (vide PL XX, figs. 2 a, 3, 4) ; while in the 

 latter they are external, and of the ordinary form. On examining another species 

 somewhat resembling them, and which I provisionally named Allorisma constrida^ the 

 cartilage fulcra appeared to occupy an intermediate position. Hence I was led to 

 consider that these shells were furnished with cartilage fulcra, varying from an internal 

 to an external position ; and that they constituted a generic group, for which I proposed 

 the name Allorisma. Moreover, I predicated of the genus a character which was dis- 

 played on several of my specimens of the so-called Hiatella sulcata, viz. a sinus in the 

 pallial line (vide PL XX, fig. 5 c). It happens, however, that the last noticed character 

 was based on insufiicient data : that, in fact, the pallial line, which I had not then seen, 

 of Sanguinolaria sulcata is entire, or, in other words, not sinuated, as was subsequently 

 pointed out by Mr. Morris.^ At the time this gentleman's discovery was published, I 

 ascertained that the Isocardia unioniformis, Phillips, possessed a pair of cartilage fulcra, 

 similar to those oi Sanguinolaria sulcata, and an entire pallial line :^ this led me to the con- 

 sideration of De Koninck's genus JEilmondia, which is typified with the former species. 

 I now ascertained that Dr. Koninck had anticipated me in the discovery of the 

 vertical cartilage fulcra, which he describes singly as " une lamelle transverse, etroite, 

 profondement situee et en partie recouverte par le crochet et ayant probablement 

 servi a supporter un ligament interne, d'une forme a peu pres analogue." It will thus 

 be evident, that in constructing the genus Allorisma, I have erred in placing together 

 two widely-different kinds of shells, — the one (with the sinuated pallial line) having been 

 inhabited by a mollusk with an anal and a branchial siphon, — the other (with the entire 

 pallial line), by a mollusk unprovided with these appendages. It is equally evident, 

 as regards the latter kind, the one represented by Sanguinolaria sidcata, that the 



— a character which was noticed in my paper (p. 316). This suspicion is now confirmed by the following 

 extract from a letter with which I have been favoured by Dr. Fleming, to whom I forwarded a couple of 

 specimens of this shell for comparison : " two of the shells do certainly resemble my Hiatella sulcata : 

 on one of the casts there are traces of the striae, a character rarely to be met with owing to the extreme 

 thinness of the shell, and its usual imperfectly preserved state." 



^ As this shell has apparently not been described, I venture to publish the following diagnosis of it. 



Allorisma constricta. King. Twice as wide as it is long. Umbones tumid ; incurved ; approximating ; 

 projecting behind the hinge ; situated a little behind the centre of the anterior half of the shell. Valves 

 with a slight longitudinal constriction immediately in front of the umbones ; transversely ribbed ; marked 

 with faint incremental lines. Ribs rather faint, and obliquely crossed with the incremental lines on the 

 anterior third of the shell ; dichotomised on the constriction ; and rather prominent on the rest of the 

 valves, except on their dorsal slope. Lunule oblong, and shallow. Corselet long, and rather deep. Dorsal 

 line slightly concave ; ventral, flatly convex ; anterior, evenly rounded ; posterior, obliquely rounded. 

 Pallial muscular sinus shallow. Found in the Carboniferous shales, Redesdale, Northumberland. My 

 largest specimen is two inches and a half wide. 



2 Strzelecki's Physical Description of New South Wales, &c., p. 270. 



3 Specimens of this shell, and other allied species, showing an entire pallial line, I first saw in the valuable 

 coal collection of carboniferous fossils belonging to the Rev. Mr. Jenkinson of Lowick, Northumberland. 



