398 THE LIAS AMMONITES. 



Dimensions.— Large shell, PI. LIII. — Transverse diameter, 200 millimetres; width 

 of umbilicus, 52 millimetres ; height of aperture, 92 mihimetres. Small shell, PI. LVI, 

 tig. 15. — Diameter, 07 millimetres; width of umbilicus, 15 millimetres; height of aper- 

 ture, 33 millimetres. 



Description. — This has been a long well-known shell of the Middle Lias. De Mont- 

 fort, who established the genus, says " the Amaltheus is still more flat than Planulifcs and 

 Mlipsolitis. Many of the petrified shells of the genus have still preserved their nacre 

 and lustre, others are ochreous and ferruginous. We find some that are more than five 

 inches in diameter. The back of the Amaltheus, instead of being rounded, is carinated 

 and armed with a crest, which renders the mouth triangular lanceolate. The specimen I 

 figure is still nacreous, and came from the environs of Anvers. This genus is very 

 numerous in fossil species ; we find them in all the cabinets, but we do not yet know 

 their living analogues, and for this reason we are obliged to place Amaltheus among sea 

 shells, which perhaps happily hazard may bring to light some other day."^ 



The shell is very much compressed and carinated ; the sides of the whorls are orna- 

 mented with slightly elevated transverse ribs, which arise straight from the sides of the 

 spiral suture, they first bend backwards and then incline forward towards the margin, 

 where they vanish ; the siphonal area is narrow, and has on its outer border a rope-like 

 prominent keel formed by a number of small short ribs which become imbricated, and 

 incline forward in a ridge-like form (PL LIV, figs. 1, 2, 3). 



The spire is composed of six whorls (PI. LITI), which are nearly two thirds involute ; 

 the outer whorl very large and embracing, leaves, however, all the inner turns of the spire 

 visible. The whorls are very much compressed, they are thickest around the umbilicus 

 and taper gently away towards the outer border. In well-preserved specimens more than 

 two thirds of the width of the whorl is covered with fine, close-set, punctated lines, which 

 follow the turn of the spire, and apparently proceed from the outer lamina of the shell ; 

 they are very well represented in PI. LIII and PI. LVI, figs. 10 and 15. The aperture 

 is very narrow and compressed (PI. LIV, fig. 1), and forms an acute angle with slightly 

 convex sides. The lobe-line is very complicated (PI. LIII, fig. 1). The siphonal lobe is 

 as wide but shorter than the principal lateral lobe, and ornamented on each side with two 

 large lateral, each subdivided into several secondary, branches. The siphonal saddle is 

 much wider than the principal lateral lobe, and terminates in five long leaves separated 

 by prominent branches. The principal lateral lobe is highly developed, consisting of a 

 central stem and several branches, two smaller internal and one larger lateral branch, with 

 a long terminal branch having several lateral digitations. The lateral is much smaller 

 than the siphonal saddle, and terminates in four folioles much divided around their 

 edges. The inner lateral lobe is much smaller than the principal lateral, and has a central 

 stem with four lateral branches, all the sides of which are much incised. The auxiliary 

 saddle is small with two terminal foliations, and the auxiliary lobes are small with sharp 

 1 ' Concbyliologie Systematique,' torn, i, p. 91, 1808. 



