PERONOCERAS, THYSANOCERAS, ETC. 229 



pointtMl. ;ind iii 11 single row. Scptal lobes with numerous 

 pointed, deeply cut, irregularly shaped minor lobes. Abdominal 

 lobe very deep, and level with superior lateral lobe. Siphonal 



cell long and narrow. 



Lower, Middle and Upper Lias. 



D. ZIPHIUS, Ziet. T. 107, fig. 611. 



PERONOCERAS. Abdomen depressed ; pilge depressed ; linear 

 between the tubercles ; usuall}', but not invariably bifurcated on 

 the abdomen. Tubercles depressed, often obtuse upon the casts, 

 but pointed and prominent upon the shell. Septa not closely 

 crowded, as in Deroceras^ or so profusely branching. 



Middle Lias. 



P. MUTICUS, d'Orb. T. 108, figs. 622, 623. 



Family THYSANOID^E. This family includes the Fimbriati, 

 Ligati, and Heterophylli, which agree in the foliaceous char- 

 acter of the septa. 



THYSANOCERAS.* Abdomen rounded ; whorls exposed ; the 

 envelopment does not extend laterally over more than one-third 

 of each interior whorl. Abdominal lobe about the same depth, 

 but narrower than the superior lateral lobe ; the latter is equally 

 divided by a peculiar minor cell of a lobiform aspect. The 

 siphonal cell is cuneiform, and the superior and inferior lateral 



cells equally divided. 



Middle and Upper Lias. 

 T. FIMBRIATUS, Sowb. T. 1()1. figs. 538, 539. 



RHACOCERAS.f Abdomen rounded ; sides of the whorls flat- 

 tened ; envelopment extends over about two-thirds of each of 

 the interior whorls, or entirely encloses them, covering up the 

 umbilicus. The lobes and cells gradually decrease in size in- 

 wardly, and are remarkable for the profusion and peculiar folia- 

 ceous aspect of the minor cells (= section Heterophylli). 



Middle and Upper Lias. 



I\. HETEROPHYLLUS, Sowb. T. 102, figs. 544, 545. 



Professor Meek includes a number of American cretaceous 

 species. 



Syn. of Lytoceras, Suess. Hyatt, Bull. Mus. Comp. Zool., iii, 59. 

 Syn. of Phylloceras, Suess. Ibid. 



