IBT.I. RUDISTID SHELLS FROM MEXICO — STEPHENSON. 7 



had fallen down into the talus, and some fragments had even been 

 washed out into the bed of the creek for a distance of a rod or two. 

 Evidently there must have been 30 or more individuals in the col- 

 ony before it was disturbed by erosion. Most of the material col- 

 lected by Messrs. Bishop and Muir is shown artificially assembled in 

 plate 3, figure 1 ; a few pieces had already been given away to inter- 

 ested persons before the photograph was taken. The locality was 

 subsequently visited by Mr. Bishop and the writer, and a few addi- 

 tional fragments were found in the talus in the gully and in the bed 

 of the adjacent arroyo. 



The second largest group of attached individuals in the colony, 

 shown in plate 1, is here designated the type of the species. It in- 

 cludes 6 moderately well preserved lower or right valves, none of 

 which, however, is perfect. No upper valves were found. The ma- 

 terial illustrated in plates 1, 2, 4, and figure 2 of plate 3 has been 

 donated to the United States National Museum by Smith, Newell, 

 and Bishop, the owners of Las Flores hacienda. 



Description.— The lower valve is large, elongate, conical, nearly 

 circular in cross section, slightly curved, slightly sinuous, or nearly 

 straight, depending upon the position of growth. All the individuals 

 in the colony are incomplete, a considerable portion of the lower ends 

 of the lower valves having failed of preservation. 



The dimensions of the largest lower valve in the group of type 

 individuals are : Length, 270 mm. ; diameter of upper end, 125 mm. ; 

 of lower end, 70 mm. ; diameter of upper end of inner cavity, 65 mm. ; 

 of lower end of inner cavity, 35 mm. 



The structural features which characterize the thick outer shell 

 layer are included in the description of the genus on pages 4-6. 



The outer surface of the shell is nearly smooth with the excep- 

 tion of distinct incremental lines, 15 or 16 weakly developed longi- 

 tudinal ribs or undulations of irregular width, and two longitudinal 

 siphonal channels separated by a prominent rib ; the anterior siphonal 

 channel (E) is shallow and narrow as compared with the broad 

 deeply excavated posterior one (S) ; both the channels and the 

 separating rib are simple, smooth undulations; some individuals 

 are nearly smooth with the exception of the siphonal areas. 



The posterior siphonal channel is a broad, even fold, and, though 

 somewhat variable in strength on different individuals, is as a rule 

 deeply excavated. 



This species differs from Tampsia chocoyensis in the markedly 

 smoother character of the outer shell surface and in the more deeply 

 excavated, smoother, and more constant posterior siphonal channel. 



Locality. — From an arroyo tributary to Arroyo el Capitan, on Las 

 Flores hacienda about 9^ kilometers north of Manuel station, near 

 the junction of the corners of haciendas Las Flores, Carrizal, and 



