218 Thirty-fifth Report o^ the State Museum. 



tending an angle of about 130°. Ears large, triangular ; posterior 

 one the larger, and denned by the abrupt slope of the side of the 

 umbo, while the anterior ear is separated by a distinct sulcus ; 

 lateral margins concave, becoming convex at the hinge-line. Byssal 

 sinus broad, rounded, well-defined and indicated on the ear by a 

 sulcus extending to the extremity of the beak. The right valve is 

 flatter and proportionally broader than the left. The limits of the 

 ears are clearly indicated by the rapid slope of the umbo, and the 

 absence of strong radiating lines of ornamentation. 



Test thin, marked by numerous regular alternating rays, which 

 increase in number by interstitial additions, and become broader 

 and stronger toward the margins. These radiating ribs are crossed 

 by very fine, sharp striae of growth. On the ears the rays are nearly 

 obsolete, and the lines of growth are sharper and stronger than on 

 the body of the shell. 



The internal mould shows a simple, continuous pallial impression, 

 extending nearly parallel to the margins, and terminating in a 

 large sub-ovate muscular impression, wnich is posterior to the 

 middle of the shell. The cavity beneath the beak extends over the 

 ligamental area at that point, forming the cartilage-pi fc. In young 

 individuals the ligamental area shows three or four small furrows at 

 the beak, extending toward the extremities of the hinge-line. 

 Three of these furrows measure 2 mm. across. In larger and older 

 individuals, this furrowed ligamental area is much more marked, 

 having frequently a width of 5 mm., and extends the entire length 

 of the hinge, showing more numerous furrows. At the beaks the 

 furrows are bent slightly outward, otherwise, with gentle undula- 

 tions, they run parallel to the hinge-line. This feature is a constant 

 character in this and several other species, and is similar to that in 

 many forms of Pterinea and Lyriopecten - . 



The dimensions of the shells of this species are very variable. 

 Large individuals have a height of 80 mm. with nearly equal length, 

 and a hinge-line of 50 mm. The graduation from this form is very 

 gradual to those in which the height is equal to, or greater than, the 

 length, and where the length of the hinge-line is nearly equal that 

 of the shell. 



This species differs from A. pecteniformis by larger anterior ears, 

 with more numerous and less prominent rays on the body of the shell 

 In specimens identified as A. SandusTcyensis the umbo is more inflated 

 and obtuse, while the length is uniformly greater than the height. 

 A. scabridus, which in some respects resembles this species, is clearly 

 distinguished by its characteristic surface-ornamentation and the pro- 

 portionally greater height of the valves. 



