Hyatt.] 268 [April 4, 



siphon near dorsum and holochoanoidal. Types, Mus. Comp. 

 Zool. This genus also includes Troch. (Lit.) trapezoidale Lossen, 

 Ueber Lit., Zeit. Geol. Gesell, 1860, p. 25, pi. 1, fig. 2, with costae 

 as in Plectoceras, and a median line of tubercles along the abdo- 

 men, but the sutures, and siphon as in this genus. 



Plectoceras, 1 nobis, includes Silurian species having costae 

 curved posteriorly on the sides and crossing the abdomen as in 

 Trocholites and sutures similar, but with ventral saddles. The 

 whorls quadrate, the abdomens narrower than the dorsum and 

 the sides convergent outwards. The siphons are ventral and 

 holochoanoidal. The young are precisely similar in form, smooth- 

 ness of the shell and striae of growth, and in sutures to the 

 straight sutured forms of Trocholites. Type, Plect. (Naut.) 

 Jason, sp. Bill., Can. Nat. Vol. 4, 1859, p. 164, Mus. Geol. Surv. 

 Can. 



Litoceras, nobis, has similar characteristics and sutures to 

 Plectoceras but the siphon is near the dorsum or below the 

 centre in the adult, and the whorl has broad abdomen and 'diver- 

 gent sides and is smooth. The young are until a late stage fre- 

 quently costated and have the siphon ventral as in Plectoceras. 

 Type, Lit. (Naut.) versutum, sp. Bill. Pal. Foss., p. 259, Mus. 

 Geol. Surv. Can. 



Diadiploceras, 2 nobis, includes species of the Devonian, with 

 costae and two rows of tubercles on the sides. Sutures have 

 ventral saddles, and in type the lower row of tubercles is 

 represented by imperfect costae in the later adolescent stages. 

 The whorl in section is quadragonal, and siphons above the 

 centre. Type, Dia. quadratum, sp. Hall, not yet described. 

 Professor Hall's Coll. Albany, 1ST. Y. Diad. (Disc.) inopinatum, 

 sp. Hall, would also answer as type, if the sutures and position of 

 siphon should prove to be similar. Nat. Hist., N. Y., Vol. 5, pt. 2, 

 suppl. pi. 110. Mus. Cornell Univer., Professor Williams' Coll. 



Metacoceras, nobis, includes Silurian and Carboniferous 

 species with broad, ventral, lateral, and dorsal lobes but no annular 

 lobes. Siphon near the ventrum or central. Whorls quadrate, 

 sides with one row of nodes along the external border, umbilical 



1 II\€KTos, twisted or plaited. 



2 ACaSiirXo's, doubled. 



