14 PALAEONTOLOGY OF CALIFORNIA. 



GENUS SPIRIFERINA, Davidson. 

 SPIRIFERINA (sp. undt.) 



THE specimen referred to the above-named genus is but a fragment of a dorsal 

 valve. It is too imperfect to exhibit any reliable specific characters, and is only 

 referred to this group from its coarsely punctate structure. It evidently belongs 

 to the plicated section of the genus, and has an extended hinge-line, being appa 

 rently somewhat like S. cristata, Schlot. (sp.) Its mesial fold is larger than the 

 lateral plications, of which there are four on each side. 



GENUS RETZIA, King. 

 RETZIA COMPRESSA, Meek. 



PI. 2, Fig. 7, and 7 a, b, c. 



SHELL small, ovate, subtrigonal, rather compressed, showing no 

 traces of a mesial fold. Beak of ventral valve moderately promi 

 nent, and but slightly arched; rounded, and truncated at the 

 extremity by a small circular foramen. Surface ornamented by 

 ten or eleven simple, rather narrow, prominent, radiating costse, 

 about equalling the deep furrows between, and extending nearly 

 or quite to the beaks. 



Length, 0.34 inch; breadth, 0.33 inch; convexity about 0.24 

 inch. 



The specimens of this species in the collection examined are not in a condition 

 to show the area, or any of the internal characters, yet they present peculiarities 

 by which it may be distinguished from any other known carboniferous species. 

 It is, perhaps, most nearly related to JR. Mormonii=Terebratula Mormonii, of 

 Marcou (Geol. North Am., pi. 6, fig. 11, a, b) ; but is a much less gibbous shell, 

 with narrower and more sharply elevated plications. It may also be compared 

 with R. punctilifera, Shumard (Trans. St. Louis Acad. Sci., vol. 1, p. 220), from 

 which it alsp differs in being more compressed, and in having fewer and more 

 prominent plications, as well as in being destitute of any traces of a mesial sinus. 



The specimens being weathered, do not show the surface punctaj very distinctly, 

 but, by the aid of a good magnifier, traces of them can be seen in a good light. 

 They are very fine and regularly disposed. 



