102 



PALEONTOLOGY OF NEW YORK. 



KuTORGA described three species of this genus, of which the foregoing is the 

 first. The others, A. disparirugata and A. recurva, vary considerably from the 

 type-species in the degree of incurvature of the high pedicle-valve, and in the 

 definition of the subapical area and the groove upon its surface, so that it will 

 be important to establish the conception of the genus strictly upon the features 

 of the first species.* Externally these shells are most closely related to Acro- 

 THELE, with which they agree in the relative positions of the umbones, and in 

 the perforation of the pedicle-valve. As observed under the discussion of that 

 genus, the distinguishing character is found in the different elevation of the 

 pedicle-valve, and the lack of any well defined subapical area in the latter 

 genus. 



Until the studies of Mr. Walcott and Mr. Matthew upon the Ameri- 

 can species of this genus were made public, nothing of importance 

 was known in regard to its internal characters. The former writer, in 

 1884,f represented a cast of the interior of the pedicle-valve of A. gemma, 

 with a somewhat broad excavation about the pedicle-aperture, which 

 would give to the interior of the valve an apical swelling or callosity, penetrated 

 by the foramen. Mr. Matthew, in 1885, gives a figure of the interior of this 

 valve in A. Baileyi,X showing the same character 

 with the addition of two pits (tubercles) close 

 upon, but just in front of the foraminal opening. 

 If this latter feature can be established it 

 furnishes another similarity to Acrothele ; but, 

 with the original specimen in our hands, we 

 must confess our inability to distinguish any- 

 thing more than the impression of the central 

 callosity (Plate III, figs. 32, 34). Mr. Walcott's figure shows elongate muscular 

 scars surrounding the foramen and diverging forward. The interior of the 

 brachial valve of A. gemma (Walcott's figure la; "ventral valve" in error), 

 shows a stout median ridge, which is widened near the apex of the valve and 



* Acrotreta reearva, Kutorga, has been taken by von Mollkb as the type of the genus Volborthia (q.v.) 



t Palaeontology of the Eureka District, pi. i, figs. 1 a-f. 



X Illustrations of the Fauna of the St. John Group, No. 3, pi. v, fig. 13 c. 



Acrotreta gemma. 



.After Walcott. 

 Fig. 55. Interior of bi'achial valve 

 Fig. 56. Carilinnl view of pedicle-valve. 

 Fig. 57. Internal cast of apical portion. 



