CEPHALOPODA. 109 



The guard is more or less elongated and compressed, sometimes conical and acumi- 

 nated towards the extremity ; at other times contracted near the apex, and enlarged rather 

 suddenly towards the alveolus ; the furrows, with which the extremity is marked, vary in 

 different specimens, both in their number, depth, and size ; there are generally two on the 

 dorsal part, one being prolonged much more than the other. The angle of the alveolar 

 cavity varies, according to M. D'Orbigny, from 20 to 25°, and is inclined towards the 

 ventral side. The aperture is generally of an oval form. 



This is a very variable species, and has consequently been described under a great 

 variety of names. M. D'Orbigny, after carefully studying a large number of specimens 

 obtained from many localities, infers that the variety of form assumed by this species is 

 mainly to be attributed to sexual differences. 



This species is generally considered to be characteristic of the Inferior Oolite in 

 England, Germany, and France ; but we have not been enabled to detect any specific 

 difference between the specimens forwarded by Mr. Bean, from the Bath Oolite of 

 Yorkshire, under the names of B. Aalensis, B. comjyressus, and typical specimens of the 

 B. gir/anteus, obtained from the Brown Jurassic formation of Wurtemberg. 



Locality. The Gray Oolite near Scarborough. Inferior Oolite of the west and south 

 of England. 1 



Ammonites macrocephalus, Schloth. Plate XIV, fig. 2. 



Ammonites macrocephalus, Schloth. 1813. Min. Taschenb., vii, p. 70. 



— — Schloth. 1820. Petref., p. 70, No. 16. 



— — Zieten. 1830. Pet. Wurtemberg, t. v, figs. 1, 4, 7; 



t. xiv, fig. 3. 



— — Quenstedt. 1843. Flozg. Wiirtemb., p. 363. 

 — If Orb. Ter. Jurrass., p. 430, pi. 151. 



— — D'Orb. Prod. Paleont., p. 297. 



? — terebratus, Phil. 1835. Geol. of Yorksb., vol. i, p. 11 6. 



after receive several corrections. At present it appears to me tbat the ' road-stone ' is characterised by the 

 great abundance of Gervillia acuta and Crassina minima, and by the presence of Pholadomya acuticostata, 

 Rostellaria composita, and the genus Actceon. Where this rock is united with the Middle Oolite, as at 

 White Nab, these fossils commonly lie near the top ; where it is entirely deficient (as at Ewe Nab), they 

 are scarcely to be found. The top of the Cave Oolite (as under Gristhorpe Cliffs, at Ewe Nab, Owlston, 

 and Ellerker) is generally marked by abundance of Millepora staminea, and plates and spines of Echini, 

 and columnar joints of Pentacrinus caput Medusa. In the substance of the rock occur Belemnites, Isocardice, 

 Pholadomya, Cuculltece, Pernce, Pinnae, Ptagiostomce, Pectines, and Terebratulce, So large a proportion of 

 its organic contents occurs likewise in the Iuferior Oolite sand beneath, that it is difficult to point out what 

 seem to be characteristic." 



1 We have had the opportunity of examining some fine specimens of this species in the collections of 

 Mr. Bowerbank and Mr. Baber. 



15 



