1892.] NATURAL SCIENCES OF PHILADELPHIA. 141 



judging from the adult characters alone, this Bacxdites is much 

 more closely related to the forms grouped under the genus Ancy- 

 loceras, and as far as the young of Ancyloceras has been described 

 it seems to be closely related to Bacxdites. On comparing the adult 

 sutures of B. compressus with Ancyloceras jenneyi Whitf, the simi- 

 larity is very marked. Lack of suitable material has prevented 

 my examining the young of Ancyloceras, but I would suggest 

 that to the genera Ancyloceras, Grioceras, and related forms with 

 completely separate whorls we are to look for the nearest relatives 

 of Bacidites. These forms, like Baculites, have become uncoiled at 

 a very early stage; their adult sutures are very similar, and the 

 main diflerence lies in the degree of straightening of the shell. 

 Indeed in Baculites the shell is not strictly rectilinear but there is 

 usually a slight curvature towards the dorsal side. While then the 

 relations of this form are still in doubt, it is hoped that the facts 

 presented in this paper may go far towards unravelling the phy- 

 logeny of Baculites. 



EXPLANATION OF PLATE IX. 



Fig. 1. Young of Bacuh'fes cor/i/>ressiis S.\y, xh). 



Fig. 2. Proioconch seen from above, x45. 



Fig. 3. Protoconch front view, x45. 



Fig. 4. Protoconch side view, x45. 



Fig. 5. Embryonic shell of first mijpionic stage, side view, x45. This is proba- 

 bly the shell possessed by the animal on leaving the egg. 



Fig. 6. Front view at the seventeenth septum, showing cross section of the 

 whorl, x2o. In this and the two succeeding figures the dotted 

 lines indicate the extent of the surface of contact between this 

 portion of the shell and the succeeding whorl. 



Fig. 7. Front view at thirteenth septum, x2o. 



Fig. 8. Front view at fourth septum, x25. 



Fig. 9. The first six suture lines, x35. 



Fig. 10. Side view of protoconch and first six septa, showing outline of the first 

 naepionic stage, x40. 



Fig. 11. Cross section of shell in the plane of the spiral showing two septa and 

 the imbricated layers of growth, xlOO. 



