431 ^ PALAEONTOLOGY OF NEW-YORK. 



moderate strength, and no transverse septum. In Spii-if era ou-cni the rostral cavity 

 is partially tilled; the dental lamellte 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 tliickcned plate or septum for one- 

 half its length from the apex. In Spirifera rmicosta the rostral cavity is tilled; 

 the dental lamellre are short and thick; the fissure is closed by a solid fillin"' 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 gregaria the rostral cavity is more or less filled with thickened dental 

 lamolhe, 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 Spin- 

 fera gramtlifera the upper part of the rostral cavity is filled with shelly matter, 

 and this ai)parcntly encroaches 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 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 cmargiuation or indentation in the apex of the cast, and a similar 

 feature is sometimes observed in the casts of other species of SriRiFERA. 



Regarding the S. granuUfera 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 lamellae, but so much thickened as scarcely 

 to merit the term septum. In Spirifera marcyi this development of shelly matter 

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

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

 medialis and >S'. macronota there is a thickened transverse septum extending from . 

 one-third to one-half the length of the fissure from the apex, and this feature is quite 

 independent of the pscudo-deltidium. The rostral cavity lies behind this septun^, 

 and may be open or free from the accumulation of shelly matter to a greater or less 

 extent; but it is not uncommon to find nothing more than a thickening of the bas« 

 of the dental plates and a partial filling of the rostral cavity. This condition 

 changes to a greater or less extent during the growth of the shell; but a partial 

 filling of the fissure at its apex is probably more common than otherwise, parti- 

 cularly in full-grown individuals. 



