On the Ascoceratidce and Lituitidce. 377 



family he recognizes four genera, viz. Ascoceras, Barrande, 1 Glosso- 

 ceras, Barr., 2 Billing sites, Hyatt, 3 and Choanoceras, gen. nov. These 

 genera are united together by a structural feature common to all of 

 them, viz. " the abnormal growth and morphology of the septa 

 formed during the last stage of their existence." Beginning with 

 regularly- formed septa, there are developed at a later stage of growth 

 a series of septa which bend upwards in a sigmoid fashion against 

 the walls of the shell, on the dorsal side. All the septa succeeding 

 the first sigmoid one are incomplete, leaving a large " lacuna " or 

 empty space (Figs, c, d) in the centre, which lacuna is bounded by 

 the lateral margins of the septa. The siphuncle (Figs. A, G, si) is 

 broad, with nummuloidal or bulbous elements. Truncation seems 

 to have been repeated several times in all these genera. 



Ascoceras. — This genus has two distinct stages in its development, 

 viz. a, the Nautiloid ; b, the Ascoceras proper. Stage a corresponds 

 with the common type of the suborder Nautiloidea. It may be 

 regarded, theoretically, as a long gently -curved tube (Fig. f), 

 corresponding in curvature with the Ascoceras part of the shell ; 

 but owing to its having been broken off or decollated several times 

 during growth its actual length seldom exceeds that of the Ascoceras 

 portion, and it often falls short of this. The test of the Nautiloid 

 stage (Fig. f, n) is generally transversely striated ; sometimes an- 

 nulated. The septa are oblique, shallowly concave, and higher 

 on the dorsal than on the ventral side : they are irregularly spaced, 

 sometimes several are placed near together, while others are wide 

 apart. The necks of the septa 4 project backwards. The siphuncle 

 is always situated near the ventral (convex) side ; it is composed 

 of slender, tubular elements. 



The second stage of Ascoceras, termed by Lindstrom Ascoceras 

 proper, is of sac- or flask-like form, slightly curved, with a 

 narrow, cylindrical prolongation or neck, ending in a simple, circular 

 aperture. 



The internal structure requires a more detailed description, es- 

 pecially as Dr. Lindstrom's researches have revealed some new 

 features in it. The septa are of two kinds : a, regular or Nautiloid 

 septa; b, Sigmoid septa. The first septum, which may be regarded 

 as the last Nautiloid septum, and forms the bottom of the shell, 

 " is strengthened from within by organic deposits of calcareous 

 matter." This septum is in some species followed by a second, 

 of the ordinary shape ; but in the majority of species the abnormal, 

 sigmoid septa (Figs, a, e, f) immediately succeed the first, ordinary 

 one. The number of sigmoid septa varies from three to seven ; but 



1 Oesterr. Blatt. fiir Litt. u. Kunst, 1847 ; Haidinger's Berichte liber d. Mittheil. 

 v. Freund. d. Naturwiss. in Wien, 1847, p. 268 ; also Bull. Soc. Ge'ol. France, 1855, 

 ser. 2, vol. xii. p. 157. 



2 Syst. Sil. de la Boheme, 1867, vol. ii. pt. i. p. 372. 



3 " Genera of Fossil Cepbalopods," Proceed. Boston Soc. Nat. Hist. vol. xxii. 

 1883, p. 278. Billingsites is a Silurian genus closely allied to Ascocems. The type 

 species is Asc. Canademe, Billings, Rep. Prog. Geol. Surv. Cauada, 1853-56, p. 3 1 0. 



4 Their recurved ends which form a little funnel through which the siphuncle 

 passes. 



