AEGOCERAS CURVICORNE. 377 



this specimen affords ; should other examples in better preservation be found the name 

 may either be retained or another specific appellation given. The Po/^»«or^/^i, according 

 to Quenstedt, exhibit many varieties, as shown in P. lineatus, P. costatus, P. interruptus, 

 P. mixtus, and P. q^mdratus, and so for the present, I have considered this species as one 

 of the group. 



Locality and Stratigraphical Position. — Found in the Middle Lias of North Lincoln- 

 shire by the Rev. J. E. Cross, F.G.S., to whose cabinet it belongs. 



Aegoceras CURVICORNE, Schlonbacli. PI. XXXT, figs. 3 and 4. 



Ammonites curvicornis, Scklunback. Eisenstein des Mittl. Lias, Zeitsch. 



Deutsch. geol. Ges., Bd. xv, p. 522, 

 pi. xii, fig. 4, 1863, 



— MACULATUS ANGULATUS, Wagener. Liasschichten Thalmulde Rheinl.West- 



phal. Verhandl, p. 166, 1860. 



— CURVICORNIS, Schlonbach. Beilrage zurPalseontologie der Jura- 



Form. Palseontographica, p. 163, 

 1865. 



Biagnosis. — Shell discoidal, compressed, with six round, bold-ribbed, slightly invo- 

 lute whorls ; sides ornamented with fifteen thick prominent ribs, which support two 

 thorn-like spines, the ribs and costae separated by wide concave valleys covered with 

 strise ; siphonal area wide and convex ; thick ribs arched forwards, passing across from the 

 marginal tubercles ; concavities deeply sculptured with fine bent striae ; aperture round. 



Dimensions. — Transverse diameter 80 millimetres ; width of the umbilicus 40 miHi- 

 metres. 



Description.— \ have long known this Ammonite from the Green Ammonite, bed of 

 Charraouth, and at one time inclined to the opinion that it was a distinct species, until 

 an example showing the transition to an enlarged body chamber at the same age as in 

 Aeg. Ilenleyi was found ; this led to a restudy of the series, and the conclusion that Aeg. 

 curvicorne is an extreme form of Aeg. latacosta, having round, depressed whorls, with 

 extremely sharp ribs and prominent thorn-like spines, and that on attaining the sixth 

 volution of its spire it commences the expansion and enlargement of the body chamber. 



Among the group of specimens of Aeg. capricorne in the Berlin Museum, V. Schlo- 

 theim has distinguished by distinct names the following varieties : — a. Am. capricornis 

 angulatus ; b, Am. spathosis ; c, Am. capricornis dorsuosus ; it is to the first of these varieties 

 that Dr. Wagener has referred the fossil now under consideration, in this opinion I am 

 inclined to concur, and regard Aeg,. curvicorne as a marked variety of the early stages of 

 Aeg. Henleyi, with depressed whorls, prominent ribs, and constricted spire ; having the 

 same expanded body chamber in the sixth volution of development as in the type form. 



49 



