372 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 DestonccHamps (1884) of young forms of ZL. 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 puncte per square 
millimetre. 
The shell at this stage is very linguloid in appearance, and recalls to mind the early 
stage of Terebratulina septentrionalis figured by Morse (Mem. Bost. Soc. Nat. Hist., 
vol. v., 1902, pl. hi. fig. 116). 
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 1°75 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 median 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 punctz 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 only under a high-power microscopic objective. 
