Clark: The San Lorenzo Series of Middle California 151 



regularly rounded. Ventral edge strongly arcuate, sloping more sharply anter- 

 iorly. Lunule large, depressed, lanceolate, about as high as wide. Escutcheon 

 long, distinct, depressed a little more than at right angles to the main outer 

 surface of the shell; it is more distinct and less coarsely sculptured, with incre- 

 mental lines stronger on the left valve than on the right. Surface of shell 

 covered by fairly coarse, somewhat irregularly spaced concentric ridges or lamellae 

 which on unweathered specimens are quite prominent, on weathered specimens, 

 sculptured by fairly fine and narrowly spaced radial ribs which are absent on the 

 escutcheon and for a short distance in front. Ventral and anterior dorsal edges 

 crenulate. Posterior dorsal edge of right valve below the ligamental groove chan- 

 neled to receive the corresponding edge of the opposite valve. Hinge plate heavy. 

 Ligamental groove deep, medium in length. Four cardinals on the right valve; 

 the posterior cardinal is low, long and narrow and is just in front of the rather 

 indistinct nymph plate; two middle cardinals are bifid, the posterior one being 

 the heavier and longer ; anterior cardinal thin, not so high as the middle two and 

 situated on the inside of the anterior dorsal margin ; on the left valve the elongate, 

 narrow posterior cardinal is set a little in front of and is separated from the 

 nymph plate by a shallow groove or socket for reception of the posterior cardinal 

 of the opposite valve ; anterior cardinal heavy and higher than the other two and 

 bluntly pointed, middle cardinal bifid. 



Dimensions. — Type specimen: greatest dorsal-ventral height, 60 mm.; greatest 

 anteroposterior length, 62 mm.; length of posterior dorsal slope, 62 mm.; length 

 of anterior dorsal slope, 16 mm. ; diameter of both valves, 39 mm. 



Occurrence. — University of California locality 1131, about three-fourth mile 

 south of Walnut Creek in basal beds of Agasoma gravidum zone. 



This species somewhat resembles Chionc Uneolata, which is found 

 in the same horizon. It differs, however, in outline, in sculpturing 

 and in the dentition. On the latter the anterior end is more produced, 

 and the posterior end is usually more angulate, the beaks are more 

 prominent, the ventral edge is not so arcuate, the lunule is longer in 

 proportion to the width and is not so depressed. The greatest differ- 

 ence between the two species is in the dental armature ; on the right 

 valve of C. Uneolata there are three cardinals which are entire, while 

 on the right valve of C. mediostriata there are four cardinals, the two 

 middle of which are bifid. On the left valve of C. mediostrkita the 

 posterior cardinal is separated from the nymph plate by a shallow 

 groove, while on the left valve of C. Uneolata the posterior tooth is on 

 top of the nymph plate. For a comparison of C. mediostriata with 

 C. cryptolineata, which is also found in the same horizon, see the 

 description of the latter. 



