LYTOCERAS JURENSE. 481 



Stephanoceras crassum, Young and Bird. PI. LXXXVI, figs. 1, 2, 8 — 10. 



Ammonites crassus, Young and Bird. Geol. Survey Yorkshire Coast, p. 253, 1822. 



— — Phillips. Geology of Yorkshire, pi. xii, fig. 15, 1829. 



— — Simpson. Monogr. on Ammonites, p. 20, 1843. 



— — Quenstedt. Cephalopoden, p, 174, tab. xiii, fig. 10, 1846. 



— — — Der Jura, p. 251, tab. xxxvi, fig. 1, 1858. 



Diagnosis. — Shell discoidal, whorls subquadrate, depressed, and slightly involute; 

 shell thick, with a circle of spiny tubercles on the margin of the siphonal area, sides orna- 

 mented with thii'ty-four costae, which commence near the umbilicus and extend to the 

 tubercle, where each bifurcates and sends two branches across the area to unite with 

 their fellows on the opposite side ; aperture subquadrate, slightly grooved below to 

 receive the return of the spire. 



Dimensions. — Transverse diameter 75 millimetres; height of last whorl 20 milli- 

 metres ; width of umbilicus 42 millimetres ; height of aperture 20 millimetres ; width 

 of aperture 22 millimetres. 



Description. — This shell exhibits similar characters to Stephanoceras commune, but 

 is a much thicker and more robust cephalopod with a deeper umbilicus. The costae 

 between the umbilicus and the tubercles are rather irregular, some are bent, others 

 straight, and the circle of spiny tubercles around the margin of the siphonal area 

 gives this variety a crown-like form. I have figured (PI. LXXXVI, figs. 1, 2) a variety 

 which closely resembles one of the varieties of Sfepli. fbulatum, and is usually collected 

 from the same shaly beds of the Upper Lias. 



ADDENDA. 



Lytoceras Jurense,^ Zieten. PI. LXXIX. 



The very fine fossil so beautifully delineated two thirds the natural size in this Plate, 

 was presented several years ago to the Museum of the Royal School of Mines by the Earl 

 of Enniskillen, F.R.S. The locality is not recorded, but I have no doubt after an exami- 

 nation of the matrix that it was obtained from the Lyt.-Jurense-zone of Dorsetshire. The 

 shell is preserved, and shows all the delicate sigmoidal curves it formed during growth. 

 This specimen I met with accidentally in one of the wall cases of the top gallery of the 



1 Seep. 413. 



62 



