AEGOCERAS BELCHERI. 313 



Aegoceras Belcheri, Simpson. PL XV, figs. 7 — 9; PL XIX, figs. 1, 2. 



Ammonites Belcheri, Simpson. Monogr. of the Ammonites Yorksh. Lias, p. 12, 1843. 

 — — Simpson. Fossils of the Yorksh. Lias, p. 43, 1855. 



Diagnosis. — Shell discoidal, compressed ; whorls narrow, numerous, from six to seven, 

 exposed, rather flattened on the sides and rounded on the outer margin ; ribs short, bent, 

 blunt, and prominent, separated by concave spaces on the sides; siphonal area rounded, 

 destitute of ribs, and marked with delicate, crescentic lines on the shell, which are 

 partly impressed on the cast. 



Dimensions. — Large specimen (PL XIX, figs. 1, 2) with shell : — number of whorls 

 6 — 8; diameter 95 millimetres; width of umbilicus 60 millimetres; height of the last 

 whorl 20 millimetres; thickness of last whorl 18 millimetres; amount of involution one 

 third of a whorl ; number of ribs on the last whorl 48. 



Small specimen (PL XV, fig. 7) without shell : — number of whorls 6 — 7 ; diameter 

 47 millimetres ; width of umbilicus 31 millimetres ; height of the last whorl 8 milli- 

 metres ; thickness of ditto 5 millimetres ; amount of involution one third of a whorl. 



Description. — The two figures I have given of this Yorkshire Ammonite agree very 

 well in comparative dimensions. The small specimen (PL XV) is entirely a cast, whilst 

 the large fossil (PL XIX) has the shell nearly all preserved. In this Ammonite the ribs 

 are rounded and directed obliquely forwards, the valleys between the elevations being 

 about the width of the ribs, which commence near the umbilical margin, maintain an equal 

 thickness on the sides, and disappear at the outer margin, leaving the round siphonal area 

 entirely smooth and ornamented only by the fine crescentic lines of the shell. In those 

 parts of the fossil where the shell is denuded we observe that the casts of the ribs are 

 much more acute than where they are enveloped by shell. The umbilicus is very wide, 

 and the growth of this interesting species was very slow. 



The lobe-line is very complicated ; PL XV, fig. 9 exhibits an enlarged drawing of 

 the lobes and saddles of the smaller specimen figured in this plate. The siphonal lobe is 

 wide, and ornamented with four denticles on each of its sides ; the siphonal saddle 

 is narrow and deep, and terminates in seven branched festoons ; the principal lateral 

 lobe, much larger than the siphonal, is symmetrical in figure, and its sides are 

 ornamented with long denticles ; the lateral saddle is as large as the principal lateral lobe, 

 and ends in five festoons ; the second lateral lobe is small, narrow, and turned obliquely 

 inwards ; the lower lateral saddle is proportionately small, and there are two small 

 auxiliary lobes with corresponding saddles of a like size. 



Affinities and Differences. — Aeg. Belcheri, Simp., closely resembles Aeg. intermedium, 

 Portl. (PL XV, figs. 5, 6), in the form of the ribs, their length, curve, and obliquity. 

 A. Belcheri wants, however, the fine elevated lines developed between the ribs near the 



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