Clark: The San Lorenzo Series of Middle California 145 



ANTIGONA (VENTKICOLA) UNDOSA, n. sp. 

 Plate 8, figures 1, 2 and 5 

 Type specimen 11136, Coll. Invert. Palae. Univ. Calif., loc. 14 



Shell subtrigonal in outline, medium in size, heavy; beaks fairly prominent, 

 strongly inturned, prosogyrous. Anterior slope straight, steep and rather short ; 

 posterior slope long and gently convex; anterior and posterior ends evenly and 

 broadly rounded; ventral edge regularly arcuate. Lunule fairly large, slightly 

 depressed, lanceolate, wider on right valve than on left. Escutcheon well defined 

 as a narrow, elongate, depressed area, more distinct on the left than on the 

 right valve. Surface of shell sculptured by numerous, fairly regularly spaced, 

 jnominent, rounded undulations, on and between which are finer incremental lines. 

 Hjnge plate rather heavy ; ligamental groove deep. Three unbind cardinals on 

 left valve; posterior cardinal rather thin and low, and fused with nymph plate; 

 middle cardinal heavy ; anterior cardinal thin and not so high as the other two ; 

 anterior lateral strong and broadly rounded. Hinge of right valve not exposed. 

 Posterior dorsal edge of right valve channeled posterior to the ligamental groove 

 to receive the corresponding edge of the opposite valve. Pallial sinus beyond 

 the middle of the shell. 



Dimensions. — Type specimen: greatest dorsal-ventral height, 54 mm.; greatest 

 anteroposterior length, 59 mm.; diameter of one valve, about 161 mm. Paratype: 

 greatest dorsal-ventral height, 46 mm.; greatest anteroposterior length, 48 mm. 



Occurrence. — A very common species in the Agasoma gravidum zone, University 

 of California localities 14, 798, 1315, 189, 1131, etc. 



Antigona undosa is of about the same size and is rather close in 

 outline to A. ncglecta; it differs from the latter in that the. anterior 

 end is slightly more produced, the lunule is not so large, the concentric 

 sculpturing is entirely different, the latter species lacking the distinct 

 undulations seen on the former. The hinge plates, also, of the two 

 species differ somewhat in minor details. 



Genus MACROCALLISTA Meek 

 MACEOC ALLIST A ? INCOGNITA, n. sp. 

 Plate 9, figures 2 and 3 

 Type specimen 11116, Coll. Invert. Palae. Univ. Calif., loc. 1131 

 Shell medium in size, broadly subovate ; beaks rather prominent, anterior to 

 the middle, strongly inturned. Posterior dorsal slope fairly long and straight ; 

 anterior slope short, nearly straight ; anterior end broadly rounded ; posterior end 

 regularly rounded but not so broad as anterior ; ventral edge strongly arcuate. 

 Lunule cordate, medium in size, marked off by a well-defined, impressed line, 

 rather strongly pouting medially. No escutcheon, but posterior dorsal margin 

 rather strongly depressed. Surface quite strongly excavated in front of beaks, 

 sculptured by somewhat irregular undulations, which in part may be due to 

 weathering; on and between these undulations are finer incremental lines. 



Din t ensions- — Length, about 30 mm. ; height, 26 mm. ; diameter of both valves, 

 14.5 mm. 



Occurrence. — So far only one specimen of this species has been found. Univer- 

 sity of California locality 1131. 



