254 
PALEONTOLOGY OP NEW-YORK. 
moderate strength, and no transverse septum. In Spirifera oweni the rostral cavity 
is partially filled; the dental lamellee 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 lamellee 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 Avithin one-third the length of the front of the valve. In 
Spirifera gregaria 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 Spiri¬ 
fera 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. 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 emargination or indentation in the apex of the cast, and a similar 
feature is sometimes observed 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 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 pseudo-deltidium. The rostral cavity lies behind this septum, 
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 base 
of the dental plates and a partial filling of theP'Yostral 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 otherAvise, parti¬ 
cularly in full-grown individuals. 
