1883.] 257 [Hyatt. 



Thus, we can say, that a given form is still gyroceran though the 

 whorls may touch in coiling, as long as the dorsum is rounded 

 and gibbous, but if the dorsum has the impressed zone, it must 

 be considered nautilian. This distinction enables us to add to 

 the peculiarities of the Silurian fauna already noted by other 

 authors (viz., the prevalence of straight, large siphoned forms, and 

 those with septa closely approximate), the additional character- 

 istic of the great raiity of true nautilian forms. 



M. Barrande has claimed that there was no approximation 

 between the ancient forms of the Nautiloidea and Ammonoidea. 



We cannot understand the facts detailed above on any other 

 supposition than the direct and independent derivation of the 

 Nautilini from a straight cone. We think farther that this 

 straight cone must have been a close ally and ancestor of the 

 straight orthoceran-like Bactrites of the Silurian. This form 

 agrees closely in all its characters with the young of the simplest 

 known forms of Goniatites. The gyroceran and tubular whorls, 

 and peculiar sutures and siphons of the young of Mimoceras are 

 very similar to those of Bactrites. The series of the Nautilinidae 

 is, therefore, similar and parallel to that of any one series of the 

 Nautiloids. It must have been independently derived from a 

 straight cone similar to Bactrites. All the remaining ammonoids 

 are more concentrated in development, and skip the orthoceran, 

 cyrtoceran, and gyroceran stages of their evolution in time. 

 They are evidently descendants of the close coiled JSTautilinidae 

 and the evidence here is very strong that the whole order of 

 Ammonoidea arose from a single organic centre of distribution, 

 the ISTautilini of the Silurian. The succession in time, the evidence 

 of gradation in structure, and the development, exactly accord 

 with this statement. Nautilinidae, Goniatites, triassic transition 

 forms of Ammonitinae and the true Ammonites of the Jura form 

 a perfect progressive series. 



The main difficulty in the way of the theory that Ammonoids 

 and Nautiloids belonged to the same stock and were derived both 

 from the same common ancestor laid in the assumed universal 

 absence of a protoconch in the latter. We have found the proto- 

 conch in several species of straight cones, and its absence in 

 others can be readily accounted for. It was a useless hollow 



PROCEEDINGS B, S. N. H. VOL. XXII. 17 DECEMBER, 1883, 



