138 



Ammonitidce sinuous or wavy. It appears probable that the mollusc added 

 periodically to the length of his shell, and formed a new chamber by throwing 

 across a partition behind him. Some Orlhocerata grew to a surprising length. 

 Woodward mentions an 0. gigantcvrn from the Carboniferous Limestone which 

 must have been, when living, 6 feet long. Dr. Grindrod has a fragment of 0. 

 Ludense 3 feet long and 7 inches in diameter. The specimen (3) in Plate i. was 

 evidently only a fragment, the septa being regularly marked throughout, and the 

 outer chamber to which the animal was restricted not appearing. 



The next noticeable feature is the "Siphuncle," a membranous tube which 

 passed through every septum, in some si)ecies centrically, in others excentrically 

 (in the Ammonitida; along the outer edge), and penetrated to the innermost 

 chamber. No satisfactory account has yet been given of the use of the chambered 

 and siphuncular apparatus. Dr. Buckland, in his Bridgewater Treatise, where 

 a clear and able account of the physiology of the Cephalopoda may be found, 

 held the view of its hydrostatic function, as enabling the animal by means 

 of air to increase or decrease his bulk at pleasure without a proportionate 

 change of weight, and therefore to rise or fall in the water. Modern natui-alists, 

 however, doubt the accuracy of this theory, firstly, because the Nautilus appears 

 on the surface only when driven by storms, and secondly, because the fossil 

 specimens exhibit the siphuncle as a continuous calcareous tube, which could 

 not have been distended. This remarkable organ varies considerably in size, 

 from one-tenth to one-half of the diameter of the shell : it presents not 

 unfrequently a series of plates iri'egularly radiating from a smaller central 

 tube. In some specimens it appears as if composed of a series of variously- 

 shaped beads, connected with the external shell by the septa of the air- 

 chambers. This may be seen in the Phragmoceras pyriforme, plate i. (4) ; but no 

 species yet found exhibits this moniliform or necldace-like structure so remarkably 

 as the unique Actinoceras baccatuni (plate ii.), fully described in the Appendix ; 

 the beads, notwithstanding their age and the enormous pressiue to which 

 they have been subjected, retaining a sphericity almost as perfect as those 

 of a new jet necklace. The great durability of this organ is attested by the fact 

 noticed by Owen (Palasontology, p. 101), that in the Silurian Umestone cliffs of 

 Dnimmond Island, silicified siphuncles of Huronia, unaccompanied by any 

 vestige of shell, have been seen standing out, like vertebral columns, 6 feet in 

 length, and Ig inches in diameter. 



The generic names of these fossils are not yet fixed detei-minately, some 

 palaeontologists being inclined to split up the genus Orthoceras into many 

 sub-genera, whUe others are satisfied with the old genus. We notice with 

 pleasure that persevering search has shown that the Canadian and Bohemian 

 genera of Dr. Bigsby and M. Barrande have representatives in the British 

 Silurian rocks : the latest discovery is the curious happily-named Ascoceras 

 (Siluria, p. 233) ; it has been found at Usk, Ludlow, and JIalvern, and we 

 must look for it at Woolhope. 



