1883.] 289 [Hyatt. 



Apsidoceratidae. 



The whorls in section are some form of the sub-triangular, and 

 throughout all the genera there is a constant tendency to sulcate 

 the abdomen and retain the gibbous character of the dorsum. 

 The genera have transverse costae, but no longitudinal ridges. 

 The marked characteristic of the group is, however, the persis- 

 tence of the large dorsal saddles, which enable us to see, that the 

 dorsal lobe is produced only in the impressed dorsal zone and in 

 this group appears to be due solely to involution. Even when it 

 is replaced by a dorsal lobe in the nautilian species, this dorsal 

 lobe becomes, as in Ephippioceras. subdivided by a small dorsal 

 saddle. 



Tripleuroceras, 1 nobis, includes straight cones, whorl in section 

 triangular, or if elliptical, having flattened abdomen. Siphon is 

 near the venter, nummuloidal, large and usually, but not invari- 

 ably, partly filled with deposits more or less radiatory in struc- 

 ture. Sutures have ventral lobe on the flat abdomen, saddles at 

 the angles, lateral lobes, and dorsal saddles or comparatively 

 straight sutures around the dorsum. One species, Trip, explora- 

 tor, sp. Bill., has an additional pair of lateral saddles. Silurian 

 and Devonian. Type, Trip. (Orth.) Archiaci, sp. Barr. Syst. Sil., 

 Vol. 2, pL 251, 480. The dorsal saddles separate this genus from 

 the Eudoceran group and show them to be the radicals of the 

 following series. 



Apsidoceras, 2 nobis, includes loosely coiled, smooth, costated 

 or tuberculated gyroceran shells, with flattened abdomens. The 

 whorls in section are triangular, the dorsum forming the internal 

 apex of the outline ; siphons near the venter and nummuloidal. 

 The sutures have broad ventral lobes, saddles at the lateral 

 angles, broad lobes on the sides, and dorsal saddles. There is 

 frequently a line of heavy tubercles on each of the lateral angles 

 of the whorls. They are all large shells and the abdomen is fre- 

 quently hollow or fluted along the centre. They occur from Silu- 

 rian to the Carboniferous inclusive. Type, Aps. (Lit.) magnifi- 

 cum, sp. Bill. Geol. Surv. Can. Rep. 1853-56, p. 307. Mus. Geol. 

 Surv. Can. 



Titanoceras,- nobis, includes Silurian and Carboniferous nau- 



1 TpiirXcvpos, three sided. 2 A\J/is, the felloe of a wheel. 



PROCEEDINGS B. S. N. H, VOL. XXIL 19 JANUARY, 1SS4- 



