GENUS SPIRIFERA. 253 



rostral cavity filled to a greater or less extent, and the dental plates 

 extremely thickened at their bases ; while there is an obscure low median 

 crest in the muscular impressions, which sometimes becomes thickened and 

 expanded above ; and occasionally there is a little filling of the upper 

 part of the fissure, resembling the incipient stages of growth of the septum. 

 In Spirifera cijdoptera the rostral cavity is more or less filled and solid, 

 with diverging dental lamellae ; while the crest dividing the muscular 

 impression is very unequally developed, and there is no appearance of a 

 septum. In Spirifera perlainellosa the rostral cavity is filled to a greater 

 or less degree, the dental lamellae are short and strong, and there is a 

 sharp elevated median crest or septum in the muscular impression ; but 

 there is no indication of the transverse septum. In Spirifera octocostata 

 the dental lamellae are thickened, and there is a vertical septum reaching 

 from the apex of the rostral cavity to the base of the muscular impression. 

 In Spirifera viarylandica of the Oriskany sandstone, there is sometimes a 

 partial filling of the rostral cavity, with a reversed conical crest in the 

 upper part of the muscular area. Spirifera arenosa has strong dental 

 lamellae ; a filling of the rostral cavity in old shells ; a low crest dividing 

 the muscular impression, which becomes thickened in its upper part; a 

 distinct pseudo-deltidial covering of the fissure, but no transverse septum. 



"Among the Spirifera of the Upper Helderberg group, Spirifera acumi- 

 nata has usually only a moderate thickening of the rostral portions, 

 .divergent lamellae of moderate strength, and no transverse septum. In 

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

 are thickened; there is a reversed conical callosity at the apex of the 

 cavity, the attenuated continuation of which 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 extends to within one-third the length of the 

 front of the valve. In Spirifera gr eg aria the rostral cavity is more or less 

 filled with thickened dental lamellae, and sometimes the incipient growth 

 of a transverse 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 encroaches upon the space 

 below with the advancing age of the shell. Tiie 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 fro-m the apex of 

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



