STREPTORHYNCHUS 71 



Brachial valve convex, most prominent in the umbonal region near the 

 hinge-line, the surface more or less compressed towards the cardinal 

 extremities, the mesial portion of the valve sometimes slightly flattened 

 or a little depressed ; the beak inconspicuous, very slightly or not at all 

 produced beyond the hinge-margin. Internally there is a broad and rather 

 low cardinal process which is flanked on either side by a widely diverging 

 socket-plate, from the outer surface of which the dental sockets are ex- 

 cavated. 



Surface of both valves marked by fine, subangular, radiating costae, 

 from three to four in the space of 1 mm., which increase by intercalation 

 and by bifurcation, crossing the costae are exceedingly fine concentric 

 markings which sometimes produce crenulations along the summits of 

 the costas. Stronger, concentric lines of growth are also frequently 

 present. 



Remarks. This little shell is one of those which exhibits a distinct 

 tendency towards an irregular growth of the shell. As it occurs in its 

 typical form the specimens have some of the characteristics of immature 

 shells, as if they might be the young of a larger form. Beede has identified 

 as the same species certain much larger shells from the fauna of the 

 Salem limestone, some of which attain a width of 50 mm. or more. These 

 larger examples are evidently identical, specifically, with the shell de- 

 scribed by Hall and Clarke as Derbya ruginosa, and if these minute ex- 

 amples are really the young of the larger ones, the name S. minutum will 

 have to give way to Hall and Clarke's prior name. However, it does not 

 seem to have been demonstrated as yet that the smaller forms are the 

 young of the larger, for they do not commonly occur in actual associa- 

 tion, and for the time being both names will be retained. The largest 

 observed examples which are certainly referable to this species do not 

 exceed 5 or 6 mm. in length. 



Horizon. Salem limestone. 



STREPTORHYNCHUS RUGINOSUM (Hall and Clarke) 

 Plate V, Figs. 1-19 



1892. Derbya ruginosa Hall and Clarke, Pal. N. Y., vol. 8, pt. 1, p. 346, 



pi. 11A, figs. 25-27. 

 1897. Derbya ruginosa Hall, 14th Ann. Rep. N. Y. State Geol., p. 345, pi. 



5, figs. 12-14. 

 1897. Streptorhynchus williamsi Weller, Trans. N. Y. Acad. Sci., vol. 16, 



p. 255, pi. 18, figs. 1-3. 

 1906. Orthothetes minutus Beede, 30th Ann. Rep. Geol. Surv. Ind., p. 



1297, pi. 20, fig. 7, (not pi. 18, figs. 1-16). 



Description. Shell of medium size or larger, transversely subelliptical in 

 outline, the thickness sometimes nearly equaling the length, both valves 

 unsymmetrically distorted and marked by strong, irregular, concentric 



