270 Mejjer — On Cretaceous Brachwpoda, 



as in Terehratula resvpinata, Sow., — and these, by projecting side- 

 ways or towards the edges of the shell, give the loop a somewhat 

 unusual appearance. See fig. ante p. 269. 



The septum of Waldh. tamarindus, except in young examples, is 

 short and but little elevated, and remains always unattached to the 

 loop. The inner surface of the shell is smooth, and never spinose, 

 as in Megerlia lima, Dav., end in the recent M. truncata, Gmel. 



In this description th«re is nothing to suggest the double (?) 

 attachment of the loop to the septum, which forms so marked a 

 feature in the section Megerlia. But Dr. Schloenbach, if I under- 

 stand him aright, has applied to the section Megerlia, the theory of 

 development of the loop and septum, which was (some years since) 

 suggested by Mr. Chas. Moore as applicable to the genus " Terehra- 

 tella,'' D'Orb. (Geologist, vol. iii., pi. xiii., '• On the development of 

 the loop in Terehratella''). And the question whether Waldh. 

 tamarindus is or is not a Megerlia depends therefore mainly on the 

 real mode of development of its loop and septum. 



In the figures given by Mr. C. Moore (see Geologist, vol. iii., 

 pi. xiii., figs. 1-4) representing several stages in the development of 

 the loop in the genus Terehratella, the loop is represented as being 

 free or unattached to the septum in the three first stages of its 

 growth, and attached only in the fourth or complete state. The 

 septum — in the same figures — is also apparently absent or un- 

 developed in the earlier stages in the growth of the loop. 



But this description, in so far as regards the absence of the septum 

 and the free condition of the loop in its earlier stages, is so entirely 

 at variance with my own observations amongst the Cretaceous species 

 of Waldheimia and Terebratella, that I cannot but suspect some error 

 of observation or delineation in the examples figured, arising 

 perhaps from the minuteness of the specimens from which Mr. C. 

 Moore's figures were obtained. 



Now I have mostly observed that the septum in the sections 

 Waldheimia, Terebratella, and Megerlia, is comparatively much longer 

 and more largely developed in the extreme young and half grown, 

 than in the adult shell, — as for instance in Megerlia lima and Tere- 

 hratella Menardi, and that it even reaches in some cases the whole 

 length of the smaller valve, as in some half-grown examples of 

 Terehratella trijida now before me. 



From these and many other examples which I could mention, it 

 would appear that the growth of the septum is at first strictly co- 

 extensive with the growth of the shell until the former has attained 

 to nearly its greatest length, and that it increases afterwards only 

 in height and thickness. And further that a septum is seldom if 

 ever produced for the first time in a full-grown shell. 



The attachment of the loop to the septum in the young of Tere- 

 hratella does not, unfortunately, so readily admit of proof owing to 

 the difficulty of obtaining perfect specimens. 



In examining the interiors of various species of Terebratella, I 

 have, however, frequently observed that the septum itself exhibited 

 what appeared to be unmistakable evidence of the continuous 



