6 PEOCEEDINGS OF THE NATIONAL MUSEUM. vol.61. 



low depression which is, as a rule, less strongly impressed than the 

 triangular depressions on either side of the sinus. 



A sinus or slit cutting the shell along the bottom of the anterior 

 siphonal channel in the manner just described, and depressions 

 on the upper rim of the shell adjacent to the siphonal channels are 

 features which, so far as I am aware, have not been found in any 

 genus heretofore described. Perhaps the nearest approach to fea- 

 tures of this kind is to be found in the genus Lapeirousia. This 

 genus, according to the diagrams illustrating it,- bears two similar 

 so-called pseudo-pillars forming a part of the inner wall of the outer 

 shell layer and corresponding respectively to the two siphonal areas. 

 Each of these pseudo-pillars is joined to the outer surface of the 

 shell by a so-called suture, which is perhaps comparable to the slit 

 or sinus in Tampsia, but Tampsia has only the one sinus and lacks the 

 psuedo-pillars. 



The genera BirodioUtes, Bournonui, and Durania, which, like 

 Tamps^ia, lack an internal ligamental ridge, present no features com- 

 parable to the sinus or slit a or the depressions on the upper rim of 

 the shell adjacent to the siphonal areas. 



Type of the genus. — Ta/nipsia hishopi Stephenson. 



TAMPSIA BISHOPI, new species. 



Plates 1-4. 



Disco vein/ and occui^'ence. — This species is based upon a colony of 

 rudistids discovered early in 1921 in reddish shale of the Mendez 

 formation by Mr. R. A. Bishop, part owner and manager of Las 

 Flores hacienda. On the basis of the rather pronounced red color 

 of the shale containing the colony, it is believed to belong to the 

 upper part of the Mendez, as that formation is developed in the 

 southern part of the ^tate of Tamaulipas; on this assumption the 

 bed is 700 or 800 feet (200-250 meters) above the base of the Mendez. 

 Since, however, the Mendez shale exhibits reddish bands at other 

 lower levels, and only a relatively small thickness of the shale is 

 exposed in this arroyo, the position of this bed in the upper part 

 of the Mendez can scarcely be regarded as conclusively established. 

 However, it is probably well above the middle of the formation. 



Dr. John M. Muir, chief geologist of the Corona Oil Co., visited 

 the locality in company with Mr. Bishop before the specimens were 

 collected from the matrix, and he says that the largest part of the 

 colony, the individuals of which still remain attached to each other, 

 was standing upright in the shale, apparently in place. Smaller 

 groups of attached individuals, separate individuals, and fragments 



»Douvill6, Henri, fitudes sur les Rudistes. Mem. de la Soc. G60I. de France, PaMon- 

 tologie, No. 41, pp. 25-27, pi. 1, flgs. 9-12, 1910. 



