70 



83. STREPTORHYNCHUS PANDORA (Billings). 



Streptorhynchus Pandora (Billings), Canadian Journal, New Series, vol. v. p. 266, figs. 

 12, 13. 



Streptorhynchus Chemungensis, var. Pandora (Hall), Pal. N.Y., vol. iv, PL IV, figs. 11,. 

 19 ; and PL IX, figs. 18, 25, and 27. 



[Compare Strophomena Wpolworthana (Hall), Pal. N. Y., vol. iii, PL XVI, figs. 

 1 and 2]. 



Shell semi-oval, about one-fourth wider than long ; the hinge-line equal to the greatest 

 width of the shell, and forming right angles with the nearly straight sides, whilst the front 

 margin is broadly rounded. Ventral valve elevated at the umbo, whence the surface slopes in 

 all directions to the margin. Area of the ventral valve large and triangular, extending the 

 whole length of the hinge-line. Foramen large, triangular, about twice as wide at the base 

 as the height, nearly or quite closed by a convex deltidium. Dorsal valve moderately con- 

 vex, slightly depressed towards the cardinal angles, with a narrow area. 



Surface marked by fine strongly elevated thread-like striae, which are sometimes alter- 

 nately large and small, and which increase in approaching the margin both by bifurcation and 

 interstitial addition. About four striae in the width of one line, sometimes more. 



" The interior of the ventral valve is marked by strong dental lamellae and a broad 

 flabellate muscular impression, which in well preserved specimens shows the cordiform im- 

 print of the adductors, and a short low mesial septum in the upper part. The interior of 

 the dorsal valve preserves deep dental sockets, with thick socket plates which support the 

 duplicate cardinal process, each division of which is grooved at the extremity. There is like- 

 wise a small process between the two branches of the cardinal process ; and below the hinge- 

 line, the flabelliform muscular impression is divided by a low rounded mesial longitudinal 

 ridge or obsolete septum." (Hall, Pal. N. Y., vol. iv, p. 68.) 



Average specimens have a width of about an inch and a quarter by an inch in length, 

 but larger individuals are known to occur. 



According to Hall, Streptorhynchus Pandora is merely a form of the vary variable S. 

 Chemungensis, (Conrad). There is, however, some doubt as to whether the specimens examined 

 by Hall really belong to S. Pandora, (Billings) ; since he states that the longitudinal striae 

 are " crenulated by fine closely arranged concentric striae." This is certainly not the case 

 with any of the specimens which have come under my notice, and Mr. Billings explicitly 

 notes the absence of concentric striae iu the examples upon which he founded the species. 

 Hall, also, states that a small centra) process exists between the two divisions of the cardinal 

 process, but Mr. Billings states that no such process exists in S. Pandora. It thus remains 

 uncertain whether the New York specimens are identical with these from Ontario. S. Pandora 

 is nearly related to the well known European species S. crenistria, from which its only import- 

 ant external difference is the absence of crenulating striae. 



Locality and Formation. Corniferous Limestone, of Port Colborne, and Hagersville. 



Genus ORTHIS (Dalman). 



" Shell variable in shape, sub-circular or quadrate ; valves equally or unequally convex ; 

 socket valve sometimes slightly concave, with or without a mesial fold or sinus ; hinge line 

 straight, generally shorter than the width of the shell ; both valves furnished with an area 

 divided by a triangular open fissure for the passage of the pedicle fibres ; beaks more or less 

 incurved, that of the larger valve generally more produced ; surface, smooth, striated, or 

 ornamented by simple, bifurcated, or intercalated ribs ; structure minutely or largely punc- 

 tated ; valves articulating by means of teeth and sockets. In the interior of the larger or 

 ventral valve the vertical dental plates form the walls )f the fissure, and extends from the 

 beak to the bottom of the shell ; between these a small rounded mesial ridge divides the mus- 

 cular scars, which extend over two elongated depressions margined on their outer side by the 

 prolonged bases of the dental plates ; the cardinal muscles appear to have occupied the greater 

 portion of the anterior division of these two depressions, the pedicle muscles occupying the 

 external and posterior part of the same space ; the adductor was probably attached to each 

 side and close to the mesial ridge. In the socket valve the fissure is partially or entirely 

 occupied by a more or less produced simple shelly process, to which were amxed the cardinal 



