﻿BEITISH FOSSIL CEPHALOPODA. 157 



example referred to the same species shows an aperture with well-marked folds 

 of growth at a diameter of 3^ inches. From the Lower Ludlow of Ludlow. In 

 the Museum of the Geological Society. 



General Description. — The section is properly circular, but is often met with 

 elliptic, which may be due either to compression or to individual variety. The rate 

 of increase in the septate portion lies usually between 1 in 9 and 1 in 12, thence 

 decreasing to almost nothing in the body-chamber. The body-chamber (fig. 4) 

 attains a length of no less than 7 inches, with a basal diameter of less than 3 inches 

 when compressed. The increase in these large examples is entirely checked, and 

 contraction takes its place. The aperture has an undulating outline ; the lines of 

 growth bending forward y the diameter on one side, and backward on the other. 

 The surface of the shell is smooth except for these lines of growth, which are 

 only conspicuous on the larger examples. The septa in the normal form are direct, 

 without any undulation ; but in some examples, not otherwise to be separated, there 

 is some amount of waving, in which case the specimens may be distinguished as 

 var. undulata. There is no real obliquity, and the convexity is from -g- to \ the 

 diameter when not increased by compression. They are distant normally T the 

 diameter, but this is not quite constant ; in mid-age they are rather closer, in 

 the young wider apart, and the last two or three may be of half the depth. The 

 siphuncle is described by Sowerby as central, though it is not quite so in the type. 

 In many examples it is accurately in the centre ; but it certainly changes position 

 without there being any other difference in the shell. When the diameters are 

 unequal, it is found on the shorter, the extreme case noticed being nearly \ only 

 from the side ; in one example it is doubly unsymmetrical (fig. 5). Its diameter 

 is \ of the whole diameter, being very slightly dilated between the septa ; at its 

 junction with which are usually some annular deposits (fig. 3). In some sections, 

 lines are observed to pass from one septum to the next ; but these are not 

 calcareous, and are merely the relics of a membranous siphuncle. The shell in 

 some examples has a thickness of 1 line, and on the cast in one case are seen 

 a large number of longitudinal risings which are not ornaments, being invisible 

 outside, but represent a kind of normal line (fig. 1). The greatest diameter 

 actually observed is a little over 3 inches. Examples of more than 2 feet in length 

 are in the Ludlow Museum. The longest actually studied is 15^ inches (fig. 1), in 

 which the body-chamber had not commenced ; other body-chambers of less diameter 

 reach 7^ inches in length, and as the smaller diameter in this example is \\ inches 

 we must allow at least 10^ inches for the smaller end. The total length of this 

 example would therefore be more than 2 feet 9 inches. The existence of still 

 larger ones may be safely asserted. The largest known septal surface, 3^ inches in 

 diameter, should correspond by comparison to a body-chamber at least 1 foot 1 inch 

 long, if it be the last chamber. Another example reduces the diameter from 3 to 2 



