410 PROCEEDINGS OF THE ACADEMY OF [Sept.,. 



d'. — Peduncle extremely short; basal margin of scutmn: 

 about equal to the width of the carina; posterior 

 cirri with more numerous spines. Capitulum 11 

 to 12.7 mm. long. Off eastern United States, 



71/. g. gracilius Pilsbry. 

 c' . — Basal margin of carina shorter than that of the scutum; 

 internal plate short, wdder than long; sides wide in the 

 lower third. Capitulum 19 mm. long. Off eastern 

 United States, .... M. annandalei Pilsbry. 

 h'. — Umbo of the carina not produced ])elow the basal margin of 

 the scutum. 

 c. — Occludent margin of scutum nearly straight; carina with 

 rather narrow sides, abruptly spreading and auriculate 

 at the base, the basal margin a straight line. Capit- 

 ulum 15.5 mm. long. Off eastern United States, 



M. rectum Pilsbry. 



c'. — Occludent margin arched; carina with wide sides and a 



large internal basal plate, its basal margin V-shaped, 



as seen from below. 



d. — Jjength of capitulum three times its breadth, the base 



narrow; scutum almost smooth. Length 14 mm. 



Off Culebra and Ascension Islands, 



M. carinaium Hoek. 



d'. — Length of capitulum twice its breadth ; scutum 



sculptured wdth spaced concentric grooves and 



fine radial impressions. Capitulum 16 to 25 mm. 



long. Off eastern L^nited States, 



M. suhcarinatum Pilsbry. 



Explanation of Plate XXXL 



Figs. 1-4. — Megalasma hdlum (Pils.). Mandil)le.s and more enlarged lower 



points of same. 

 Fig. 5. — M. helium. Maxilla. 

 Fig. 6. — M . minus Annand. Maxilla. 



Figs. 7, 8. — M. minus. Mandible and its lower point more enlarged. 

 Fig. 9. — M. minvs. Outline of primitive shell and adjacent margin of the 



scutmii. 

 Fig. 10. — M. 77)inus. (iioup of ova. 



