STREPTORHYNCHUS. 31 



Such are the general dispositions presented by the shells composing this sub-genus, 

 and which denote its intermediate position between Orthis and Strophomena. Professor 

 King lays great stress upon the peculiar twisted character of the beak, but it remains still 

 to be ascertained whether this can be considered a common feature of the species which 

 compose the group, or peculiar only to some specimens. 



Now, if we compare the shell above described with those of D'Orbigny's Orthesina, 

 of which woodcuts figs. 4, 5, and 6 are faithful representations, 1 the differences will be 

 obvious. It will be seen that both valves possess large areas, with convex deltidiums, 

 leaving no open or free space whatsoever between them at the hinge-line (woodcut, fig. 4), 

 but in that of the larger valve, and towards its extremity, is seen a circular or oval 

 aperture, which is in general cicatrized or closed in full-grown shells, but which was open 

 up to a certain age, and evidently affording passages to pedoncular fibres of attachment. 

 No similar aperture ever existed in Streptorhynchus, for no trace of such a foramen can be 

 seen in any of the species and numerous specimens that have come under my observation, 

 while it would appear to be a constant character in Orthesina anomala, 0. Verneuili, 0. ad- 

 scendens, &c. ; and if any fibres of attachment ever existed in Professor King's sub-genus, 

 they must have passed through the small space left between the deltidiums and the cardinal 

 process of the ventral and dorsal valves, a point not hitherto satisfactorily established. 



In the interior of the larger valve of Orthesina there exists, as in Streptorhynchus, a 

 strong hinge-tooth on either side at the base of the fissure ; but instead of the small or 

 rudimentary dental plate or ridge peculiar to the last-named sub-genus, in Orthesina these 

 last are largely developed, and of a remarkable shape : they first project considerably into 

 the interior, and converge towards the extremity of the beak (woodcut, fig. 5 n), and 

 afterwards bend slightly outwards, and inwards, to form a third curved and connecting 

 plate (o). This last is supported by a vertical mesial septum, which extends along the 

 bottom of the valve to about half its length. On either side of this last are faintly-marked 

 impressions, probably due to the cardinal muscle. Towards the extremity of the beak, on 

 the under surface of the deltidium, is seen the foraminal aperture (/), surrounded by a 

 raised margin ; and the inner surface of the dental plates (n) probably afforded a surface 

 for the attachment of the pedicle muscle, which must have been large, from the extent of 

 the grooved surface. No such characters can be found in the larger valve of Strepto- 

 rhynchus. 



The smaller or dorsal valve of Orthesina has also its well-marked and peculiar features. 

 Its large area and convex deltidium have already been described. At its base, on either 

 side, are situated two sockets for the articulation of the teeth of the ventral valve : the 

 cardinal process (j) is very remarkable ; it is formed of a single projection, with two small 



1 I must express my warmest thanks to Count Alex. V. Keyserling for the kind exertions he made to 

 procure me the means of studying the interior of Orthesina ; and also to Professor Dr. Schmidt, of 

 Dorpat, for the two beautiful interiors of Orthesina anomala, which he kindly gave me, and from which the 

 woodcut representations, figs. 5 and 6, have been taken. 



