DR. T. DAVIDSON ON RPXENT BEACIUOPODA. 175 



APPENDIX. 



Atretia Brazieri, Davidson. (Plate XXV. figs. 10-17 a.) 



Atretia Brazieri, Davidson MS.; .\gnes Craue/Proc. Zool. Soc. 1880, p. 18.'5. 



[Note. — This species was received by Dr. Davidson, and named in manuscript sliortly before his death. 

 A short description was published by me to secure priority for the name Atrelia Brazieri, wliich 

 Dr. Davidson had given it, after Sir. John Brazier of Sydney, who dredged eleven specimens in the 

 harbour of Port Stephens, New South Wales. — A. C] 



This pretty little Brachiopod presents all the well-marked characteristics of the genus 

 Atretia. Two short, curved, slender processes, denticulated at their extremities, descend 

 from the small narrow hinge-plate of the smaller dorsal valve, and an elevated wedge- 

 shaped projection rises abruptly from the central mesial se])tuni of the same valve : the 

 presence of this septum is indicated by a dark line visible from the exterior of the shell. 

 The shell is small, generally longer than wide, triangular in shape, especially in the 

 younger specimens. Dorsal valve rounder than, and not so large as, the ventral one, 

 slightly convex, flcxuons towards the centre at the margins ot the valves. The ventral 

 valve, owing to the prolongation of the beak-area, is longer and more triangular than the 

 dorsal one, raised towards the beak, which is somewhat produced and incixrved, with a 

 triangular foramen commencing beneath its pointed extremity. Two elevated ridges 

 extend from the shoulder of the shell nearly to the margins of the valves, and there seems 

 to be a slight elevation corresponding to the well-marked external depression, and sur- 

 rounded by muscular scars, in the interior of the two specimens I have examined under 

 the microscope. The shell is shallow towards the margins, but rounded and deeper near 

 the beak. Shell-substance imperforate. Surface smooth, glossy, and gleaming, marked 

 with fine concentric lines of growth, semitrans})arent, horn-coloured, or light grey. 

 Length '1\ lines, width 1^, depth 1 line. Another specimen measured 2 lines in length 

 by 2j lines in width ; this was more flattened and depressed, and the external mesial 

 sinus in the ventral valve was less naarked. Other specimens were aljout 1 line in length. 



When examined under microscopic power the scaly structure of the shell is very 

 apparent ; there are no perforations. The circular and elongated sac-like aggrega- 

 tions apparently result from the partial overlapping of the cycloidal shell-scales. The 

 presence of parallel rows of spicular projections was revealed in the interior of the shell ; 

 these recur at regular distances from each other, running from the l)eak to the margins 

 of the valves. 



Hah. Eleven specimens and odd valves were obtained by Mr. John Brazier off Cabbage- 

 Tree Island, Port Stephens, New South Wales, in 25 fathoms, on sandy mud. 



Obs, Five of these specimens Mr. Brazier forwarded to Dr. Davidson in July 1885. 

 Dr. Davidson examined them, made drawings, and named the species after Mr. Brazier, 

 to whom he was indebted for much information concerning the recent Australian 

 Brachiopoda, as he has frequently stated in this monograph. 



