114 PALEONTOLOGY OF NEW YORK. 



the margin, with rarely intercalated finer ray's; crossed by regular, sharp, 

 elevated, concentric lamella' which (in good specimens) are produced into Bub- 

 tubular, spiniform extensions upon the rays. Lines of growth are seen 

 between the lamellae. On the wing the rays are more subdued while the 

 concentric lamellae are strong. The ear is marked only by the crowded con- 

 centric striae. On the right valve the radii are obsolete on the body and 

 well-marked on the wing, and the lamellose expansions are conspicuous. In 

 some casts they appear as undulating elevated lamellse. 



Pallia] line extending parallel to the margin of the shell and terminating 

 in a muscular impression on the posterior slope. A small muscular impres- 

 sion is also seen just in front of the beak, and obscure indications of one or 

 two cardinal teeth. 



Ligamental area narrow, striated, marked by two or three slender grooves, 

 which are slightly divergent from the hinge-line. 



One of the original specimens of A. quadrula {=A. Boydi) has a length of 

 30 mm., height and hinge-line each 28 mm. A similar specimen has a 

 length of 25 mm., height and hinge-line each 23 mm. 



Two large specimens belonging to the group of A. Boydi have the follow- 

 ing respective dimensions : length 38, and 40 mm., height 38, and 35 mm., 

 hinge-line 34, and 30 mm. 



This species, in older specimens, resembles A. subdecussata, but differs in the 

 character of the rays, in the less obliquity, and the wider body. The left valve 

 somewhat resembles the right valve of Avicula recticulata, Hisinger, but a com- 

 parison of the right valves of both species shows them to be different. 



The forms here classed under one specific designation were described by Mr. 

 Conrad under different species, viz.: Avicula Boydii, A. quadrula, A. pleuroptcra, 

 and probably A. jicrobliqua. 



'flic species in its various stages of growth and state of preservation presents a 

 greal variation in form and surface characters. The usual condition is that of casts 

 of the interior or specimens more or less exfoliated, leaving the surface markings 

 subdued or obsolescent, and in a few specimens only, have the entire characters 

 of the test iu cither or both valves been observed. The older specimens show 



