LYTOCERAS TORULOSUM. 417 



Lttoceras TORULOSUM, ScJiuMer. PI. LXXVI, figs. 4, 5. 



Ammonites torulosus, Schubler, in Zieten. Versteiner. Wurtemb., p. 19, tab. xiv, 



%. 1, 1830. 



— toviViVho^vs Keferstein. Naturgeach., ii, 414, 1834. 



— TORrLOSus, d'Orbigny. Terr. Jurass., i, p. 322, pi. 102, figs. 1, 2, 6, 1842. 

 _ _ Quenstedt. Flozgeb. Wurtemb., p. 287, 1843. 



— — Quenstedt. Cephalopoden, i, p. 104, tab. vi, p. 99, 1849. 



— — Morris. Catalogue of Brit. Fossils, p. 295, 1854. 



— — Quenstedt. Der Jura, p. 306, tab. 42, fig. 7, 1858. 

 Lytoceras tortjlosum, Neu7nayr. Zeitschrift Deutsch. geol. Gesells., Bd. xxvi', 



p. 89.3, 1875. 



Diagnosis. — Shell discoidal, one third involute ; whorls round, and transversely ribbed ; 

 costae nearly straight, broad, round, and separated by wide valleys ; siphonal area round 

 with very large ribs ; shell thin, ornate, with numerous transverse striae ; umbilicus wide, 

 ribs becoming attenuated near the spiral suture. 



Dimensions. — Transverse diameter, 80 millimetres; height of aperture, 35 milli- 

 metres; width of whorl, 30 millimetres. 



Description. — This fine Cephalopod has a discoidal shell shghtly compressed and about 

 one third involute. The sides and siphonal area are covered with nearly straight, round, 

 prominent ribs, about from thirty to forty in number, according to age ; on the last whorl 

 they are separated by deep concave valleys, which impart a distinctive character to the 

 ribbing of this shell. On the area the ribs attain their greatest thickness and the valleys 

 their greatest depth, whilst near the umbilicus and around the spiral suture they are much 

 attenuated and closely approximated. The shell is extremely thin and seldom preserved ; 

 when it exists, the outer lamina is found to possess a series of long, delicate striae, which 

 run along the ribs, and these are very well delineated on the area (PI. LXXVI, fig, 5). 



The siphonal area is wide, round, and very convex; the ribs are larger and the 

 valleys deeper in this region than on the sides (figs. 4 and 5). The spire is composed of 

 five or six roundish or ovate whorls, which are nearly one third involute. The aperture is 

 oval and grooved below by the return of the spire. 



The shell is extremely thin, about the thickness of drawing paper, and its outer 

 lamina is covered with narrow, sharp striations, two or three of these adorn and follow 

 the line of the ribs, and a portion of this structure is preserved on the siphonal costae of 

 the fragment delineated in fig. 5 of PI. LXXVI. Most of the examples I have examined 

 appear to have broken ofi" at the line of junction of the body-chamber with the last septum. 

 One specimen, a compressed fossil from Mossingen, in a brownish shaly clay, has much of 

 the shell preserved, showing upon the convex sides of the ribs from two to four sharp, 

 well-defined striae, but in none of the examples has the sutural line been observed. 



AJfinities and Differences. — Lytoceras torulosum in many respects resembles Lytoceras 



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