of Terebratulina septentrionalis. 419 



consequent greater density. They were probably the clivari- 

 cator muscles, though somewhat in advance of what their 

 position would be in the adult state. 



Owing to the contraction of the digestive sac, its diverticular 

 cavities were sharply defined, and the globular appearance of 

 the lower cavity was marked. In another alcoholic specimen 

 of the same age, a band, evidently one of the lateral gastro- 

 parietal bands, was made out. As this could not be verified 

 in other specimens of the same age, the figure is not given, 



A still more advanced stage is shown in fig. 12. The shell 

 is now becoming proportionally broader ; and the cirri in- 

 crease in number, though still forming a simple circle around 

 the mouth. The crura have also begun to form. 



In fig. 13 a considerably more advanced state is shown. 

 The valves have been forcibly separated, and the smaller one 

 is thrown back. The cirri are more numerous, numbering 

 thirty-one ; two of them are seen encroaching upon the cir- 

 cular lophophore ; and at this stage the lophophore has begun 

 to assume its hippocrepidan character. 



The crura [cr] are plainly seen supporting the crown of 

 ciiTi ; and the liver already shows the first indications of its 

 differentiation into the peculiar cgecal ramifications which 

 become so numerous in the adult j and at this stage is seen 

 the division of each lateral portion of the liver into a dorsal 

 and a ventral lobe. The liver is divided into a series of cgeca, 

 though these are united. 



The divaricators {d) are completely formed ; and between 

 these two muscles is seen the lengthened intestine, the 

 blind extremity of which is held firmly to the shell by a 

 membrane, called by Hancock the ventral mesentery. 



The appearance of the shell at this stage is represented in 

 fig. 16. Radiating ribs, to the number of fifteen, ornament 

 the shell. It will be seen by this figure that the setge corre- 

 spond in growth and position to the radiating fun-ows ; and 

 this observation is also made by Hancock in his examination 

 of the adult. A well-defined concentric line seems to indicate 

 the earlier XtViywZa-shaped shell ; and though no ribs appear in 

 the earlier stages, they are defined upon this area afterwards ; 

 and it would appear from this that the ribs are formed on the 

 inner surface, and that the setae direct and induce the fui-row. 

 The lobes of the liver (/) and the peduncle {p) and peduncular 

 capsule are shown in this figure. 



In fig. 14 a stage still more advanced is given ; the valves 

 are thrown back, disclosing the stomach and intestine sus- 

 pended from the calcareous loop. The crura {or) are well 

 defined; and the ventral mesentery (r 7??) is more distinctly 



