250 THE LIAS AMMONITES. 



Inferior Oi>lite. 



Harpoceras Murchisonse, Sow. I Harpoceras Sowerbyii, Mill. 



— Tessoniauum, d'Orb. \ — Edwardianum, d'Orb. 



Genus Oppelia, Waag. — Shell discoidal and highly involute ; umbilicus very 

 small ; whorls much elevated. Ventral edge either more or less acute, or rounded, on 

 different portions of its extent. Sculpture slightly falciform, with a double set 

 of ribs on the sides. Body-chamber sometimes ribbed, neither keeled nor angular; 

 from one half to two thirds of a whorl in length. Mouth-border falciform, sometimes 

 with auricles, always having round external lappets. Siphuncle large, with a calcareous 

 sheath, Aptychus thick, calcareous, bivalved, and folded. Adductor muscles situated near 

 the border of the shell in the lower half of the whorl. Suture-line extremely ramified. 

 Siphonal lobe mostly shorter than the principal lateral^ which is large and much branched. 

 The second lateral is in like manner well developed, and there are several large auxiliaries 

 between the second lateral and the umbilicus. Lobe-bodies small, with almost parallel 

 borders ; lateral lobes unequally branched. The genus Oppelia appears first in the 

 Lower Oolite, zone of Cosmoceras Parkinsoni, as Oppelia subradiafa. Sow. ; and the last 

 representative of the group, as far as our knowledge at present extends, is in the Upper 

 Jura of Stamberg, where a considerable number of different forms are found. 



The English species of this genus are not numerous. Oppelia subradiafa, Sow., from 

 the Inferior Oolite, is a good example. Dr. Waagen has figured a number of beautiful 

 forms of Oppelia, amounting to twelve species, from the Golden Oolite of Keera Hill, 

 near Charee, Kutch, and other higher Oolitic Beds. " Oppelia subcostaria, 0pp., closely 

 resembles the European 0pp. subradiata, not only in form, but even by similar varieties, 

 as in Europe, in the same stratum " (Waagen). 



Genus Haploceras, Zittel. — The forms included in this group have been separated 

 from the genus Oppelia, as they present certain characters in common by which they 

 differ from the typical lines of that genus ; they are all distinguished by a narrow umbilicus, 

 a smooth surface, marked with fine lines of growth, like Hapl. 

 ooliticum, d'Orb., from the Inferior Oolite, with its thin undivided 

 falciform sculpture passing round the shell. Sometimes there 

 are a series of straight parallel ribs at intervals, as in Hapl. ligatum, 

 fig. 156, between which a number of very fine, undivided, smaller, 

 and thinner folds are placed. The ribs are not bifurcated in this 

 genus, and there is neither a keel nor a channel in the siphonal 

 area. The body-chamber is short and the structure of the 

 Fig. \56.— Haploceras Aptyckus uukuown. The number of the lobes is variable. In 

 iga urn, r . addition to the siphonal lobe and the principal and smaller laterals. 



