102 
PALAEONTOLOGY 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,\ 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 gemma. 
After Walcott. 
Fig. 55. Interior of brachial valve 
Fig. 56. Cardinal view of pedicle-valve. 
Fig. 57. Internal cast of apical portion. 
* Acrotreta recurva, Kutorga, has been taken by von Moller as the type of the genus Volborthia (q.v.j 
f Palaeontology of. the Eureka District, pi. i, figs. ,1 a-f. 
I Illustrations of the Fauna of the St. John Group, No. 3, pi. v, fig. 13 c. 
