﻿86 BEITISH FOSSIL CEPHALOPODA. 



tlieir own distance ; there are no intermediate ribs, but there were five transverse 

 lines to each rib, as seen on the external cast. The septa are parallel to the ribs, 

 and are therefore 3° oblique, and occupy the re-entering angle between them, and 

 are therefore distant ^ the diameter. The siphuncle is unknown. The diameter is 

 f inch. From the Lower Silurian Shales of Desertcreat. In the Museum of 

 Practical Geology. 



General Description. — The name adopted for this species belongs to a specimen 

 which forms the most outlying member of a very variable group, the more typical 

 members of which have been referred to various other species resembling, but not 

 identical with this ; hence the general idea of the species is rather different from 

 that given by the specimen described as type. In none is the true form of section 

 seen, all being elliptic by compression. The rate of increase in one example com- 

 mences with being 1 in 9, but, with age, the rate decreases to almost zero ; the 

 rapidly expanding form, figured by Portlock, being contorted. No good characters 

 of the body-chamber are observable. The ornaments consist of — first, transverse 

 undulations, which are of very variable character, being sometimes wider than the 

 interspaces, as in Portlock's type, sometimes narrower, as in the specimens figured on 

 Plate III., but tending to become less acute and conspicuous with age ; they are 

 from 4° to 1 2° oblique, and occupy ^ to ^ of the diameter : secondly, a cancellation 

 over the surface, varying in character with age. Up to a diameter of 8 lines there 

 are longitudinal sharp riblets, about -|- line apart, all, at first, of equal size, and with 

 smaller intermediate ones, crossed by sharp transverse lines about 5 per line, but 

 the latter, from the state of preservation, are not always visible, and even the inter- 

 mediate longitudinal ones disappear ; next the surface passes through an intermediate 

 stage, in which the longitudinal riblets are in some places alternate, and in others 

 in groups (fig. 5a), i.e. a larger one with several of irregular size between (fig. 9c), 

 at the same time the transverse lines become more widely spaced, and divide the 

 area into squares (fig. 9b) ; finally we get the form figured by Portlock, in which 

 the larger longitudinals are clearly marked with three between each (fig. 9a), and 

 the transverse lines are regular. These changes un fortunately have to be traced from 

 specimen to specimen, but Portlock recognised the smaller ones as the young of his 

 figured example, and they have the same general aspect. It is thus seen that 

 Portlock's type represents a form in which all but the strongest longitudinal lines 

 have died out, and these have become of more importance than the transverse ribs. 

 The septa are in each hollow, cutting the upper slope of the rib in the middle, and 

 are thus -^ diameter apart. The siphuncle is apparently central. The greatest 

 diameter seen is 1^ inches. 



Relations. — The various names that have been assigned to the specimens, here all 

 referred to one species, show the great variability of that species, and this must be 

 reckoned one of its characters. In Sowerby's 0. tubicinella, from the Devonian, the 



