150 J. WILFRID JACKSON ON 



dredged at this station, range in size from 1 mm. to 7 mm., and furnish ample material 

 for a study of the gradual development of the shell and brachial support. 



Though the development of the latter organ is well known in the genus Liothyrina, 

 through the careful studies by Deslongchamps (1884) of young forins of L. vitrea, it 

 may be of interest to give here a detailed description of the various stages in the develop 

 ment of this appendage, based upon a study of the young examples in my possession. 



In the smallest example (L., 1 mm.) the brachial support has only just commenced 

 to make its appearance, and is represented by two small sharp points * descending 

 from the rudimentary crural bases, which consist of two short raised diverging bosses 

 bordering the dental sockets. The apical portion of the ventral valve of this specimen 

 shows a somewhat triangular peduncular opening, which is slightly notched on each 

 side. No deltidial plates are apparent, but the teeth are fairly well-developed. The 

 shell-mosaic of both valves is quite clear, but irregular in its development. The shell- 

 perforations are large and well rounded, and show on an average 256 punctaj per square 

 millimetre. 



The shell at this stage is very linguloid in appearance, and recalls to mind the early 

 stage of Terehratulina i>eptentrio7ialis figured by Morse (Mem. Bost. Soc. Nat. Hist., 

 vol. v., 1902, pi. li. fig. 16). 



At 1 '5 mm. in length the shell has assumed a more pear-shaped outline ; the 

 descending branches of the loop have increased slightly in length and diverge strongly 

 from each other. The peduncular opening is more normal in shape, and traces of 

 deltidial plates are slightly visible. 



At 175 mm. the deltidial plates are still further developed and the descending 

 branches of the loop exhibit slight traces, near their bases, of the crural points. 



At 2 "5 mm. the branches of the loop are curved slightly backwards and inwards 

 towards the bottom of the valve ; they are here more ribbon-like in form than in 

 previous stages. The deltidial plates show increased development ; the shell-mosaic is 

 very irregular and wavy or flow-like in arrangement. 



At 3 and 3'5 mm. the crural points exhibit greater development and the loop 

 branches show a stronger convergence towards each other. 



At 4 mm. the converging branches are almost in contact at their extremities, but 

 no angle, as yet, is present on their surface. 



At 5 mm. the deltidial plates are larger and seen to be highly punctate, and the 

 cardinal process of the dorsal valve has appeared. The loop is still unjoined, and there 

 is no sign of angulation. In the umbonal cavity of both valves a thin dark line is 

 apparent (visible through the shell) which probably represents the meilian septum. 

 The mosaic of the muscular impressions in the dorsal valve is clear and scale-like, but in 

 remainder of the shell is very irregular. The punctae in this specimen range from 224 

 to 280 per .square millimetre. 



At 6"5 mm. the characteristic angulation, at the junction of the descending branches 



* Visible rinly miller a high-power microscopic objective. 



(ROY. soc. EDIN. TRANS., VOL. XLVIII., 372 ) 



