Contributions to Invertebrate Palaeontology. 523 



MOLLUSCA. 



LAMELLIBRANCHIATA. 

 Genus MYTILAUCA H. and W. 



Prelim. Notice Lamellibranchiate Shells, Up. Held., Ham. and Chemung Groups, etc. 

 State Cab. Nat. Hist., Dec. 1869. 



Mytilarca percarinata. 



Plate X, figs. 1 and 2. 



Mytilarca percarinata Wliitf., Ann. N. Y. Acad. Sci., March, 1882, p. 202. 



Shell less than medium size, the specimen used for description and illus- 

 tration measuring but one and three-fourths inches in extreme height ; and 

 the distance from the anterior to the posterior margins across the point of 

 greatest diameter, only a trifle over one inch ; the depth of the valve being 

 nearly half an inch. Form of the shell elongate triangular-ovate, rather 

 acutely pointed at the beak, which is small and incurved ; anterior, or byssal, 

 margin straight and absolutely vertical in the example mentioned ; basal 

 margin broadly rounded from the anterior line nearly to the point of greatest 

 length of the valve, where it is more rapidly curved, and finally passes abruptly 

 into the rajjidly ascending posterior margin ; the lower part of which is nearly 

 parallel to the anterior side, but above inclines more rapidly toward the short 

 and very oblique hinge-line. The surface of the valve is most elevated along 

 the anterior umbonal ridge, where it is at right angles to the anterior surface, 

 but slopes gently backward for two-thirds of the distance toward the posterior 

 margin, and on the other third much more abruptly. Near the beak, the sur- 

 face rounds rapidly from the anterior ridge to the posterior border. Surface of 

 the shell marked by numerous concentric ridges, parallel to the margin of the 

 valve, many of which are strongly marked and form varices of growth. On 

 the anterior surface, these varices and the concentric striae are well marked. 

 Cardinal area not observed. 



The example used is a right valve, and bears evidence in its cha- 

 racters of being an adult shell. It is associated in the same layers 

 of cherty material with M. ponderosa, H. & W. (Prelim. Notice 

 Lamell. Shells, etc., p. 21), but may be readily distinguished by the 

 vertical anterior surface and the angular umbonal ridge. From the 

 young of that species, it is readily distinguished by these characters, 

 as those are distinctly round and ventricose. The only known 

 species approaching this in the angularity of the ridge, is M. atten- 

 uata, H. & W., of the Chemung group; but this is quite distinct 

 in other respects. 



Formation and Locality. — In the white chalky chert-beds of the 

 Upper Helderberg Group, near Dublin, Ohio. 



