AEGOCERAS TAYLORI. 349 



Diagnosis. — Shell discoidal ; vvborls rounded and externally compressed ; sides with 

 fourteen straight, elevated, upright ribs, with two large, blunt, lateral tubercles, one on the 

 side and one on the siphonal area ; the ribs separated by wide concave spaces. Siphonal 

 area concave and bi-tuberculate ; aperture rotund,, two fifths the diameter of the 

 shell. 



Dimensions. — Transverse diameter 50 millimetres ; width of umbilicus 20 milli- 

 metres; height of aperture 18 millimetres ; transverse diameter 20 millimetres. 



Description. — This beautiful Ammonite is a very rare British fossil, and was very 

 well figured by Sowerby ; it appears to be more common in Germany where it has been 

 collected from its true horizon by Professor Quenstedt, who has given good figures of 

 the two varieties of this species found by him. Aeff. Taylori nodosus is the typical form, 

 and is the one I have collected at Lyme Regis, and figured in PI. XXXI, fig. 5. This 

 variety has four series of blunt, well-marked, prominent tubercles around the whorls ; 

 those on the sides appear like a thickened extension of the ribs (fig. 5), which contract 

 above and unite themselves with the prominent nodules (figs. 6 and 7), developed around 

 the lateral region of the siphonal area (fig. 6). When these tubercles were covered with 

 shell they constituted very prominent processes and formed the Am. lamellosus of 

 d'Orbigny (Ter. Jurass., PI. 84). The specimen figured in PI. XXXI, fig. 5, is simply 

 a mould and conveys no idea of what this highly ornamented Ammonite was when it 

 possessed its shell entire. 



The second variety, Aeg. Taylori costatus, had in early life very sharp prominent ribs, 

 on which the side tubei'cles were scarcely developed ; those on the margin of the 

 area appear as the termination of the ribs, rising high up on each side of the concave 

 furrow, which the siphonal area forms between the two series of tubercles. This 

 rare variety is very often mistaken for other forms. The ribs are separated by 

 wide concave spaces covered with striae, which describe the natural curve of the ribs ; 

 as they wind over the margin, and pass across the siphonal area, they become more 

 and more developed (figs. 6 and 7). 



The aperture is rotund, slightly grooved on the outer margin by the turn of the 

 spire, and rather longer longitudinally than transversely, as is well shown in fig. 7. 



The lobe-line is complicated ; the siphonal lobe is formed of two branches of equal 

 parts, composed of several lateral digitations with two terminal bifid digits. The siphonal 

 saddle is large with several folioles at its termination ; the principal lateral lobe as long 

 as the siphonal, is narrow and elongated, with four bifid lateral digitations on each side, 

 and a long terminal digit. The lateral saddle is also wide, terminating in two bifid and 

 one trifid foliole ; the lateral lobe is a small representative of the principal. The accessary 

 lobes appear to be well developed, still their details are concealed in the tui'n of the spire. 



Affinities and Differences. — This species resembles the young condition of Aeg. 

 striatum, yet the central furrow along the siphonal area and the row of tubercles on the 

 sides thereof form good diagnostic characters, by which it may be easily distinguished 



