CONTRIBUTIONS TO PALEONTOLOGY. 291 



occasionally there is a little filling of the upper part of the fissure, resembling 

 the incipient stages of growth of the septum. In 8pirifera cycloptera 

 the rostral cavity is more or less filled and solid, with diverging denial 

 lamellae; while the crest dividing the muscular impression is very unequally 

 developed, and there is no appearance of a septum. In Spirifera 'perlarmllosa 

 the rostral cavity is filled to a greater or less degree, the dental lamella are 

 short and strong, and thei'eis a sharp elevated median crest or septum in the 

 muscular impression; but there is no indication of the transverse septum. In 

 Spirifera ootocostata 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 marylandica of the Oriskany sandstone, there is some- 

 times 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 acuminata 

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

 lamellte of moderate strength, and no transverse septum. In Spirifera oweni 

 the rostral cavity is partially filled; the dental lamellte are thickened; there 

 is a reversed conical callosity at the apex of the cavity, the attenuated continua- 

 tion 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 lamelUe are short and thick; the fissure 

 is closed by 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 gregaria the rostral cavity is 

 more or less filled by the thickened dental lamellse, 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 shell}' 

 matter, and this apparently 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 exten- 

 sion 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 sub-conical 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 emargination or 

 indentation in the apex of the cast, and a similar feature is sometimes observed 

 in the casts of other species of Spikifeea. 



"Eegardiug the S. granulifej-a alone, there is little to attract especial notice, 

 beyond the general fact of a partial filling of the rostral cavity with the exte- 

 rior portion prolonged between the dental lamellse, but so much thickened as 



