NATURAL SCIENCES OF PHILADELPHIA. 223 
Genus MACROCERAMUS, Guild. 
Jaw thin, almost membranous, semi-transparent, light horn 
colored, strongly arched, ends acuminated; cutting margin without 
median projection; anterior surface with numer- 
ous delicate, separated ribs, denticulating both 
margins; these ribs run obliquely towards the 
median line of the jaw, so that the central ribs 
meet before reaching the lower margin of the jaw, 
forming an upper median triangular space be- 
tween the ribs. 
It was formerly considered that this jaw was 
actually in separate pieces, whose overlapping 
margin formed the ribs upon the anterior surface 
(see fig. 65). More careful examination, however, 
Jaw of Macrocera. a8 proved the jaw to be in one single piece, with 
mus signatus. delicate ribs upon its surface. 
ee There are over 50 ribs on the jaw of the only 
one of our species I have examined, M. Gosset. I givea copy of 
Mr. Bland’s figure of the jaw of J. signatus, which is similar. 
The lingual membrane of Macroceramus was supposed to be 
the same as in Cylindrella described above, as that of JZ. segnatus 
was so found by Mr. Bland (Ann. Lye. Nat. Hist. N. Y., VIII., 
162), and Crosse and Fischer (Journ. de Conch., 1870, pl. III., fig. 
14-16). It was, therefore, with surprise that I found an entirely 
different type of dentition in JZ G'osset. I can in this place only 
note the difference, and leave to future study the question of 
its bearing on the generic position of the species. 
M. Gossei (pl. XV., fig. 1) has a membrane very long and narrow; 
teeth about 40—1—40, in scarcely oblique transverse rows, deci- 
dedly not en chevron. Centrals with a long, narrow base of at- 
tachment with somewhat expanded lower lateral angles, its upper 
margin squarely reflected. ‘The reflected portion is very small, 
and bears three short, blunt cusps, the median the largest, all 
three with distinct cutting points. The base of attachment of the 
laterals is long and narrow, its outer lower angle irregularly cut 
away; the upper margin broadly and obliquely reflected, the re- 
flected portion thrown off obliquely towards the margin of the 
lingual membrane, very short and bearing two stout, blunt, short 
cusps, the inner the larger, also thrown obliquely towards the 
