100 Annals of the Carnegie Museum. 



on a platform made when they unite with the median sejjtum is the 

 scar of the pedicle muscle. The entire interior of the valve, outside 

 the muscle scars, is covered with small pustules (Fig. 17). 



S. demissa Hall. 



Hall, Pal. N. Y., IV, 1867, p. 81, pi. 11, figs. 14-17, pi. 12, figs. 1-5. 



This species is represented by two or three poorly preserved ventral 

 valves. None are sufficiently complete to give measurements of length 

 and width. The hinge structure is fairly well shown. The deltidium 

 is broad and convex, and the pedicle opening is just at the beak. One 

 specimen shows well the course of the pedicle. The rudimentary teeth 

 are supported by a median septum formed by the coalescence of the 

 two dental lamellae. Between the teeth and running to the beak is a 

 groove, covered over by the deltidium, thus forming a pedicle passage. 

 The median su])port is very short and in front of it is the scar of the 

 pedicle muscle. 



The diductor scars are very large, flabelliform and bounded by two 

 diverging ridges which are widely separated posteriorly. The adduc- 

 tors form a rather large oval scar which is divided longitudinally by 

 a raised line. Toward the front of the scar are two faint diagonal 

 lines which mark off the anterior adductors. They are very much 

 smaller than the posterior pair and acuminate in front. The median 

 septem is not strongly developed in front of these scars. 



Orthothetes ' chemungensis Conrad. (Plate V, Rows i, 2.) 

 Hall, Pal. N. Y., IV, p. 67, pi. 10, fig. 6. 



In this collection there are about four hundred specimens of Ortho- 

 thetes re[)resenting all stages of growth from a size of .71 X .80 mm. 

 to 16 X 22 mm. Well preserved large specimens are uncommon and 

 the majority of the individuals are less than 10 mm. in width. On 

 the other hand, specimens less than 2 mm. in width are common. 

 There are in this material two species, one of which, O. cheniungensis, 

 is represented by two varieties, O. cJieimiii^ensis arctistriatus Hall 

 and O. che/>iungensis pecienacea Hall. Most of the material can be 



1 In a paper on "New Molluscan Genera from the Carboniferous" (Proceedings 

 of the U. S. National Museum, Vol. XXVII, page 721, 1904) Dr. Girty states 

 Miat Orthotetes {sic) as defined \>y Fisher-de-Waldheim covers the type of structure for 

 which Waagen proposed the name Derbya. Thus, according to Girty, Derbya be- 

 comes a synonym for Orthotetes, and he suggests Schucliertella, for shells of the 

 type of Streptorhynchus lens White. 



