1866.J 249 I"*"- 



Among the Spirifers of the Upper Keldcrberg group, Spirifera 

 acuminata has usually only a moderate thickening of the rostral por- 

 tions, divergent lauielh^ of moderate strength, and no transverse septum. 

 In Spirifera oweni the rostral cavity is partially filled; the dental 

 lamella) are thickened ; there is a reversed conical callosity at the 

 apex of the cavity, the attenuated continuation of wbich divides the 

 muscular area : the fissure is closed by a thickened plate or septum 

 for one-half its length from the apex. In Spirifera raricosta the 

 rostral cavity is filled; the dental lamellae are short and thick; the 

 fissure is closed by a solid filling for a part of its length, while from 

 the bottom of the cavity there rises a thin vertical septum which ex- 

 tends to within one-third the length of the front of the valve. In 

 Spirifera gregaria the rostral cavity is more or less filled with thick- 

 ened dental lamelliB, and sometimes the incipient growth of a trans- 

 verse septum is perceptible. 



In several of the Spirifers of the Hamilton group the septum is 

 well marked, while in others there is an incipient development of the 

 same character. In Spirifera granulifera the upper part of the 

 rostral cavity is filled with shelly matter, and this apparently en- 

 croaches upon the space below with the advancing age of the shell. 

 The fissure becomes partially filled, and a thickened plate projects a 

 little in advance of the solid filling of the beak, leaving behind it, or 

 on the inner side, a conical cavity directed towards the apex of the 

 shell. The muscular area is divided by a low longitudinal crest, 

 which, in its extension towards the beak, becomes wider, and is often 

 much thickened towards its junction with the solid shelly matter 

 filling the rostral cavity. Looking at this feature from the apex of 

 the shell, it presents a subconical form, and is more or less abruptly 

 attenuated towards the centre of the muscular impression. This 

 callosity sometimes becomes so prominent as to produce an emargi- 

 nation or indentation in the apex of the cast, and a similar feature is 

 often obs^ved in the casts of other species of Spirifera. 



Regarding the S. granulifera alone, there is little to attract 

 especial notice, beyond the general fact of a partial filling of the 

 rostral cavity with the exterior portion prolonged between the dental 

 lamellas, but so much thickened as scarcely to merit the term septum. 

 In Spirifera marcyi this development of shelly matter has the char- 

 acter of a true septum, closing one-third or more of the length of the 

 fissure from above, and leaving an open rostral cavity behind it. In 

 Spirifera medialis and S. macronotus there is a thickened transverse 



