NATURAL SCIENCES OF PHILADELPHIA. 



223 



Fig. 65. 



Jaw of Macrocera- 



mus signatus. 



[Bland.] 



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, 

 has proved the jaw to be in one single piece, with 

 delicate ribs upon its surface. 



There are over 50 ribs on the jaw of the only 

 one of our species I have examined, M. Gossei. I give a copy of 

 Mr. Bland's figure of the jaw of M. signatus, which is similar. 



The lingual membrane of Macroceramus was supposed to be 

 the same as in Cylindrella described above, as that of M. signatus 

 was so found by Mr. Bland (Ann. Lye. Nat. Hist. N. Y., VIII., 

 162), and Crosse and Fischer (Journ.de Conch., 1810, pi. III., fig. 

 14-16). It was, therefore, with surprise that I found an entirely 

 different type of dentition in M. Gossei. 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 (pi. 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 



