AEGOCERAS PLANORBIS. 309 



Ammonites psilonotus l^vis, Quenstedt. Cephalopoden, p. 73, pi. iii, fig. 18, 



1849. 



— — Quenstedt. Handbuch der Petrefactenkunde, tab. iii, 



fig. 18, p. 354, 1852. 



— erugatus, Simpson. Fossils of Yorkshire Lias, p. 42, 1855. 



— planorbis, Oppel. Juraformation, p. 73, 1856. 



— psilonotus L.EVIS, Quenstedt. Der Jura, p. 40, 1858. 



— planorbis, Dumortier. Depots Jurass. du Bassin du Rhone, 



torn, i, p. 28. 1866. 

 Aegoceras planorbis, Tate and Blake. Yorkshire Lias, p. 270, 1876. 



Dimensions. — Figured shell: diameter 85 millimetres; height of the last whorl 21 

 millimetres ; width of umbilicus 47 millimetres. 



Diagnosis. — Shell depressed, discoidal ; whorls, seven to eight, slightly involute ; sides 

 covered with fine, straight, hair-like striae, which are well seen on the outer lamina of the 

 shell, and faintly visible on the cast ; siphonal area narrow and rounded ; aperture 

 oblong, with a well-marked circular contraction ; Anaptychus horny. 



Description. — This Lias Ammonite is found nearly always crushed between shaly 

 laminae. Sowerby's original specimens from Watchet were in that condition, with 

 the mother-of-pearl lamina well preserved. The natural form of the shell, however, is 

 well shown in my figured specimen from the Yorkshire coast, and in another fine example 

 from Wiirtemberg. The test is smooth, compressed, and discoidal, and the outer 

 lamina of the shell is covered with fine, straight, hair-like lines of growth (PI. XIV, 

 figs. 1 — 3), which sometimes develop into folds ; the siphonal area is round and smooth 

 (fig. 2), and the body-chamber contracted near the aperture by a narrow oval band, 

 which appears around that opening (fig. 3). The whorls, seven in number, increase 

 very slowly in height; they are slightly involute, and the inner whorls are all fully 

 exposed. The aperture is oblong and compressed on the sides (fig. 2) ; and the shell, 

 almost as thin as paper, is preserved in part on both of the figured specimens, which 

 have escaped compression. In nearly all the Planorbis-heds of England the delicate 

 shells are crushed quite flat, and many of the points described above cannot be seen. In 

 some of the fossils from Lias blocks washed up in Robin Hood's Bay, the casts of this 

 Ammonite occur in a semi-transparent state, in which the sinuous lobe-line is beautifully 

 displayed. The siphonal lobe (fig. 4) divides into two branches, with five serrations on 

 each side ; the siphonal saddle is about the same size, with five festoons ; the principal 

 lateral lobe is elongated and narrow, with asymmetrical digitations ; the principal lateral 

 saddle is larger than the siphonal; the lower lateral lobe is nearly as large and 

 asymmetrical as the principal lateral ; the auxiliary lobes are small and multidigitate. 

 The lobe-line is very well seen in fig. 1, and an enlarged accurate drawing of the lobes 

 and saddles is given in fig. 4. 



Affinities and Differences. — In a recent important memoir, 1 Dr. M. Neumayr has added 

 1 'Zur Kenntniss der Fauna des untersten Lias in den Nordalpen,' p. 24. 1879. 



