428 NATURAL SCIENCE. dec. 



the protoconch, do occur in these beds. The large size and 

 unshrivelled condition of the protoconch is supposed by Clarke to 

 be due to the fact that it may have been " derived from a shell 

 so young that atrophy and wrinkling have not manifested themselves 

 as they may have done with the more mature development of the 

 shell." 



The caution of this writer is so extreme that it is a little difficult 

 to understand exactly what he does think, and it would be hazardous 

 to give his conclusions in any words than his own. " If," he says, 

 " the evidence presented brings us to the conclusion entertained by 

 many of the older palaeontologists, that Bactrites is closely related to 

 Ovthoceras, this conclusion is attained, by means of data not before 

 elaborated, namely, the existence in both of like protoconchs. This 

 fact, fortified by the decisive evidence that in Badvitcs the sipho is 

 strictly intra-marginal, the formation of a dorsal lobe wholly casual, 

 leads to the conviction that Bactrites is little else than an orthoceran 

 nautiloid with a lateral sipho. Both Branco and Hyatt have sug- 

 gested the probability of Belemnitcs having been derived from ortho- 

 ceran stock. Hyatt demonstrates that the guard or rostrum in this 

 genus is a hypertrophic secondary deposition about the earlier parts 

 of the true conch, similar to the plug which is sometimes found 

 filling the distal fractured extremity of the sipho of orthoceratites. 

 The similarity of structure in the true conch of Beleinnites, Bactrites, 

 and Orthoceyas, is now increased by the demonstration of like proto- 

 conchs in all." 



It is not clear whether Clarke would deny the goniatite affinities 

 of Bactrites, or at least ally it more closely to the nautiloid Orthoceras 

 than he would to any ammonoid ; or whether he believes in the 

 existence of a straight protoconch-bearing ancestor for each order, 

 Bactrites being the ammonoid and Orthoceras the nautiloid ancestor, 

 and the coleoid ancestor being as yet doubtful. The latter view, or 

 something like it, seems to me to be nearer the truth ; but I fear it is 

 the former that is held by Clarke. It is therefore well to point out 

 that his new observations do not necessarily support that view. If 

 the protoconch which he figures in Bactrites does not so closely 

 resemble that of Mimocevas compressum as the one figured by Branco, 

 at all events the whole initial part of the shell, with its '■^ Oncocevas- or 

 Gomphoceras-like swelling of the shell-tube directly above the proto- 

 conch," bears a very striking resemblance to the young oi Agoniatites 

 fecundus.^ Of course no stress can be laid upon the backward direction 

 of the septal necks, for, as this is the direction in the ammonoid young, 

 it is also the direction that we should expect to find in the ammonoid 

 ancestor. The lateral, even if not marginal, position of the siphuncle 

 is a bit of positive evidence strongly in favour of the ammonoid 

 affinities of Bactrites. The sculpture also reminds one of the more 

 primitive goniatites rather than of a nautiloid. The evidence of the so- 

 2 Barraiide. "Systeme Silurien," vol. ii., Suppl, pi. 490, fig. ii., i. 



