408 THE LIAS AMMONITES. 



are only slightly involute. The shell is thin and ornamented transversely with from eighty 

 to one hundred elevated striae, the free margins of which in perfect specimens are fimbriated 

 as shown in PI. LXXI, and between these striae, as they cross the siphonal area, a number 

 of smaller intermediate striae are introduced. Besides these regular fimbriated annular 

 striae the outer whorl exhibits from eight to twelve prominent plates, which stand boldly 

 outwards and embrace the entire circumference of the volutions. These remarkable 

 processes are finely preserved and carefully cleaned out in the splendid specimens figured 

 in Pis. LXXI and LXXII. 



The siphonal area is well rounded (PI. LXXII, fig. 3), and partitioned off by the 

 wing-like foliations ; the body-aperture (PI. LXXII, fig. 2) is oblong, only slightly grooved 

 by the last turn of the spire ; the sides are a little compressed, and the lip is thickened 

 by an oral band shown to be a product of the mantle, which periodically put on increased 

 formative activity and developed annular bands of shell-growth eight or ten times during 

 the evolution of the last whorl. 



The sutural line is very complicated, and the siphonal lobe (PI. LXXII, fig. 4) is much 

 smaller than the principal lateral. Each stem has four lateral branches, of which the two 

 posterior are the longest and most digitate. The siphonal saddle is small and divided into 

 two portions by the multidigitate character of the lobes. 



The principal lateral lobe is very large and springs from a thick stem which divides 

 into two large branches, each of which subdivides into a number of digitations producing 

 an arborescent character in this well-developed lobe, as shown in PI. LXXII, fig. 4. 



The principal lateral saddle is much smaller than the lobe and formed of two 

 unequal parts by a central secondary lobe, the external portion is the largest and most 

 ramified. 



The internal lateral lobe is about half the size of the principal, and like it has a central 

 stem bifurcating into two branches which subdivide into several smaller branchlets. 



The accessory lobe is formed of small oblique digitations, and the corresponding 

 saddle is proportionately wider than the others, where it rests upon the involution of the 

 spire. 



The evolution of this species is only imperfectly known. D'Orbigny stated that 

 when a young shell had attained a diameter of 13 millimetres and retained its test, he 

 had noticed transverse striag and indications of laminas. In older shells these characters 

 became more developed ; in one shell, 30 millimetres in diameter, the specific 

 characters were well shown, and the lateral striae were very small ; the vertical laminae 

 on the sides were inclined obliquely forward, and were vertical across the area. 



The mould when entirely denuded of its shell is smooth ; at those parts where laminae 

 have existed, depressions instead of prominences, may be observed ; these mark the 

 former successive terminal apertures of the body-chamber. 



Affinities and Differences. — This species has been often confused with Lytoceras 

 cornucopia. In Lyt. fimhriatum the whorls are narrower and compressed on the sides, 



