ALLOEHYNCHUS 201 



of R. ricinula are not infrequent, and their association is such as to in- 

 dicate that they are without doubt the young of the larger examples 

 which more typically represent R. macra. These two names were both 

 proposed by Hall in the same paper, and as ricinula occurs on an earlier 

 page it might be given priority by some, but as no injustice can be done 

 by adopting macra it seems best to do so since that name was applied 

 to the mature and more normal form of the species. 

 Horizon. Salem limestone. 



ALLOBHYNCHUS ACUTIPLICATUM n. sp. 

 Plate XXIV, Figs. 83-86 



Description. Shell small, broadly subovate in outline, broader than 

 long, the greatest width anterior to the mid-length of the shell, the 

 postero-lateral margins nearly straight and meeting at the beak in an 

 angle of about 90 degrees or a little more, the antero-lateral margins 

 rounded, the anterior margin usually nearly straight but sometimes slight- 

 ly sinuate or a little convex. The dimensions of a perfect specimen are : 

 length of pedicle valve 10.9 mm., length of brachial valve 9.8 mm., greatest 

 width 12.7 mm., thickness 7.2 mm., width of sinus in front 6.9 mm. 



Pedicle valve convex in the umbonal region, the surface curving abrupt- 

 ly to the postero-lateral margins and a little inflected to the cardinal 

 extremities, from the umbo to the antero-lateral margins the surface is 

 gently convex and from the beak to the anterior margin along the 

 median line it is arched with the strongest curvature anteriorly; the 

 mesial sinus originates near the middle of the valve and becomes rapidly 

 deeper towards the front, where it is a little produced in a broadly 

 rounded extension, its surface is nearly flat transversely in the bottom, 

 with the sides sloping outward; the beak is sharply pointed and only a 

 little incurved, being produced posteriorly conspicuously beyond the 

 incurved beak of the opposite valve, with its apex somewhat remote from 

 the umbo of the opposite valve ; the delthyrium broadly triangular, only its 

 basal part filled by the beak of the opposite valve, the remaining portion 

 closed by the deltidial plates which are sometimes destroyed in the fossils ; 

 the plications simple, fine and subangular towards the beak, becoming 

 acutely angular towards the front margin, four usually occupying the 

 sinus, all of which may be included in the bottom of the depression or one 

 may be crowded up a little way upon one of the lateral surfaces, from 

 eight to ten occupy each lateral slope of the valve, growing successively 

 smaller towards the cardinal extremities, the last three or four becoming 

 very faint and sometimes nearly obsolete; the concentric markings of 

 the valve are obscure or nearly obsolete, consisting of a few faint lines 

 of growth which are commonly present near the margin of the valve. 

 Internally the dental plates are well developed and extend one-fourth 

 or a little more of the length of the valve anteriorly from the beak. 



