of Terebratulina septentrionalis. 42S 



at the base, while the remaining portion is made up of scales 

 similar to those composing the shell. From an exti'emely 

 young specimen I observed these same peculiar acicular scales 

 (fig. 26) ; and they probably indicate the presence of the crura. 



Nothing of a satisfactory nature was made out regarding 

 the structure of the shell in the earliest stage {fig. 2, PI. XV.) — 

 though, had the shell possessed the peculiar scale-like struc- 

 ture and cajcal perforations, they would have been visible. 



Groivth of the Shell. — Figs. 9, 10, and 11, PI. XV., severally 

 show portions of the hasmal valve, neural valve, and the valves 

 connected, from an early Linguliform stage. In fig. 9 the 

 hinge-plate (Ap) and the dental socket [ds) are well marked ; 

 in the earlier stages of their development the crara appear as 

 slight projections. In fig. 10 the prominence of the so-called 

 teeth of the shell [te) is shown. 



Fig. 11 represents the valves joined ; the caeca are seen 

 perforating the entire margin of the shell about the foramen, 

 which is very wide and rudimentary. 



On PI. XVl. a series of stages are given to show the gradual 

 development of the crura and parts sm*rounding the foramen. 

 The upper line of figures in this series, marked N, represents 

 the neural or ventral valve ; and the lower line of figures, 

 marked H, represents the hasmal or dorsal valve. In the 

 youngest of these stages, figs. 28 and 29, a broad gap in the 

 neural valve indicates the peduncular foramen. It will be 

 seen that the shell is folded upon each side of the peduncular 

 passage. These folds never meet in this species. 



Figs. 30 and 31 show a slightly more advanced stage, the 

 difiference being mainly in the greater prominence of the 

 crura. 



In the next stage, figs. 32 and 33, the calcareous loop is 

 well advanced, though not yet connected by a continuous 

 shelly layer, although at all stages these portions are connected 

 by a membrane charged with spicula. Figs. 34 and 35 repre- 

 sent similar portions of an adult shell for comparison. It will 

 be seen that the neural process of the loop becomes connected, 

 while the haemal process does not become so connected, though 

 in some specimens it closely approximates. The peduncular 

 opening becomes more circular; and the cardinal process {cp)y 

 which does not appear in the earlier stages, is now present. 



In the early stages the shell is as long as it is broad, and 

 the hinge-margin represents the greatest transverse diameter 

 of the animal. Later the shell elongates, with the sides 

 nearly parallel, and the peduncular opening wide and gaping. 

 In a stage intermediate between the Linguliform and the adult 

 the peduncular opening is more contracted, the sides of the 



32* 



