112 



INFERIOR OOLITE AMMONITES. 



Remarks. — In this genus are included species ribbed in two different ways, 

 namely, the straight and the sigmoidal. The former are distinguished from Lillia 

 by the absence of knobs on the inner area, and by the concave inner margin ; the 

 latter are separable from Ludivigia by the ribs being simple (not bifurcate) and 

 inconspicuous on the inner area; while the peculiar suture-line, with its broad 

 siphonal lobe, short and broad superior lateral lobe placed so near the inner margin, 

 and the other lobes undeveloped, fully serves to distinguish the genus from any 

 of those previously mentioned. 



The species which are included in this genus may be arranged as follows : 



a. With sigmoidal radii : — Hildoceras bifrons (Brug.), H. boreale (Seebach). 



b. With transitional radii : — Hild. Levisoni (Simpson). 



c. With straight radii : — Hild. Kiliani, Haug, H. Douvillei, Haug. 



Dr. Haug also includes Am. serpentinus (Reinecke), and Am. Frantzi (Reynes), 

 in the " group of Am. bifrons}" 



The species of this genus appear to be directly descended from Am. algovianus, 

 Oppel, or some form very similar thereto. They seem to show a transitional state 

 of ribbing between the Arietitan and Harpoceratan styles. Their general shape 

 and suture-line are decidedly inclined to the former style. 



The species of this genus are characteristic of the Upper Lias, in fact so much 

 so that the presence of a Hildoceras may be looked upon as an almost infallible 

 guide regarding the date of the deposit containing it. 



Hildoceras bifrons (Bruguiere), var. Plate XXII, fig. 30, 31 ; Plate A, fig. 28. 



1867. Ammonites bifrons, Meneghini. Fossiles calcaire rouge (Lias superieure), 



Paleontologie Lombarde, 4 e serie, p. 8, 

 pi. i, figs. 5 a, b, c. 



I have thought it unnecessary to enter into the synonyms of this well-known 

 species. The one reference which I have made to Prof. Meneghini's work 

 illustrates a specimen having the greatest resemblance to the varieties to which I 

 wish to draw attention, and showing a considerable difference from the specimens 

 figured by d'Orbigny, Wright, and others. Hild. bifrons, like the other species of 

 the genus, is intimately associated with the Upper Lias ; and the only reason that 

 I introduce the species here is because I have found this variety in great 

 numbers in an indurated band towards the lower part of the Cotteswold Sands, — 

 strata which, classed as " Midford Sands " on the supposition that they were 



1 " Beitrage Monogr. Harpoceras," ' Neues Jahrbuch f. Min., &c.,' Beil.-Bd. iii, p. 642. 



