ENDOTHYRA. CEPHALOPODA. 17 
become more deep and quadrangular, thus producing a new external contour 
in the shell. 
To elucidate this subject still farther, we have given a representation 
of Ammonites Murchisone, plate I. fig. 18. It will be noticed, that the 
inner volutions in the infant state, as seen in the innermost volutions, are 
smooth ; and afterwards in the young condition, the volutions are crossed by 
pretty strong, curved ribs, which are somewhat irregular in their degree of 
elevation. These continue until the shell has reached the diameter of about 
two inches, when they very suddenly become plain, exhibiting only transverse 
and slightly waved lines of growth. 
Amiunonites seem not to have been subjected to the same laws of geographical 
distribution as other animals, which existed along with them in the ancient 
world, for we find them widely spread in almost every country of the globe— 
the same species in strata of the same age, extending throughout Europe, 
Asia, and North and South America, Dr Gerard detected, in the year 1830, 
at an elevation of 16,000 feet, at Thibet, in the Himmalaya Mountains, speci- 
mens of Ammonites \Valcotii, plate ILI. fig. 26, and A. communis, which were 
identically the same with those found at Lyme Regis in Dorsetshire, and 
Whitby, Yorkshire. The former of these species also occurs in the Lias of 
Normandy, and south of France, Befort, Haut Rhin, Boll and Ackelberg, 
and also in the Volitic group of strata. The Greensand of New Jersey, like 
that of England, contains Ammonites, Hamites, and Scaphites. Captain 
Beechy and Lieutenant Belcher detected Ammonites on the coast of Chili, in 
lat. 36, S. in the cliffs near Conception. 
This former universal diffusion of identical species, in organic life, differs 
remarkably from the existing distribution of animals and plants; for we find 
the geographical range of species limited with astonishing exactness, to certain 
parallels. Hence it seems certain, that during the Secondary and Tertiary 
periods, a more general diffusion of the same species prevailed than at present, 
through regions of the world most remote from each other. 
Naturalists have divided the Ammonites into several sub-genera, depending 
chiefly upon their external form being more or less globular, the structure of 
their spiral convolutions, being furnished with single, double, or quadruple 
caring ; and their external surface being knobbed, ribbed, or plain; but these 
are not so much to be depended upon as the sinuosity, number, general 
structure, and distribution of the septa. 
Genus XI1.— ENDOTHYRA. — Phillips. 
Generic Character. — Shell involute, discoidal, internally 
concamorated, the chambers communicating by a large per- 
foration ; the septa arranged in stellated order; their 
emarginations on the inner part of their disk; destitute of 
any shelly siphuncle. Form of the septal edge unknown. 
Size, one-fiftieth of an inch. 
Endothyra Bowmani. Plate VI. fig. 2. Found in the 
Mountain Limestone of Westmoreland. 
TRIBE II.— NAUTILACEA. 
Shell disciform, with a central spire, and short cells, which 
do not. extend from the centre to the circumference. 
B 
