114 PALAEONTOLOGY OF NEW YORK. 



till' iiiartiiii. with rarely intercalated finer rays; crossed by regular, sharp, 

 elevated, com;entric lamelltc which (in j^ood specimens) are produced into sul)- 

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

 between the lamelhv. On the \\ in^ the rays are more subdued while the 

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

 centric striiv. On the right valve the radii are ()l)solete on tiie IhhIv and 

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

 some casts they appear as undulating elevated hiniellai. 



Pallial line extending parallel to the niari:in 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 sleniler grooves, 

 which are slightly divergent from the hinge-line. 



One of the original specimens of A. quadrula (=^-4. 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 2o 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 3-> mm., 

 hinge-line 34, and 30 mm. 



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

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

 somewhat resemliles the right yalve of Avicula recticulata, Hisinger, but a com- 

 parison of the right valves of both species shows them to lie diU'erent. 



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

 Conrad under difterent species, viz. : Avicula Boydii, A. quadrula, A. pleuroptera, 

 and probably A. perobliqua. 



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

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

 ol"the interior or specimens more or less exfoliate<l. leaving the surface markings 

 sul)(lueil or olisolescent, and in a few specimens onl\'. ha\-e the entire characters 

 of the test in either or Imih \al\es lieen observi'il. The older specimens show 



