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dorsal saddles in place of dorsal lobes, as in the sutures of their 

 nearest allies among the Nautilini and all of the remaining Ammo- 

 noidea. Mimoceras ambigena Barn, of the Silurian (Figs. 7, 8, 

 PI. ii), is a close ally of this Devonian species, and with Mimoceras 

 (Gon.) lituwn (sp. Barr.) Hyatt (Figs. 40-42, PI. viii), are the only 

 ammonoids which are not involute nautilian in form. The whorls 

 are in contact ; but there is no impressed zone, and no sutural lobes 

 on the dorsum, as in true nautilian shells. On the contrary, they 

 are purely gyroceran forms, with rounded dorsum and sutural sad- 

 dles on this side in place of lobes. All of the Nautilinidae also have 

 the septa concave, as in the Nautiloidea, in place of the invariably 

 convex character of the septa in later Ammonoidea, as shown in 

 PL x. As doubts may disturb the mind as to whether M. compres- 

 sum is an ammonoid, we recommend a comparison of this shell 

 with the young of an undoubted species of Goniatitinse, Agonia- 

 tites fecundus of Barrande, which is a miniature copy made by her- 

 edity (Figs. 9-1 1, PI. ii). 



Bactrites is a perfectly straight form, similar to the members of 

 the Goniatitinse in all important characteristics, especially the siph- 

 uncle and septa, and it also has, like the young shell described by 

 Clarke and all the coiled Ammonoidea, a comparatively large proto- 

 conch, as demonstrated by Branco, whose figure has been repro- 

 duced on PI. 2 of this paper. This same genus includes straight 

 cones like Bactrites (Orthoceras) pleurotomus Bar. {Syst. si/., PI. 

 296), which are undeniably transitions to true Orthoceras in their 

 striae of growth and position of siphuncle. There is, therefore, 

 convincing evidence in the structures of these Silurian shells that 

 the Ammonoidea, with their distinct embryos, arose from the 

 orthoceran stock, and passed through a series of forms, in times, 

 perhaps, preceding the Silurian, which were parallel to those char- 

 acteristic of a number of genetic series among Nautiloidea, viz., 

 straight, arcuate, gyroceran, and nautilian. 



In Science (Vol. iii, No. 52, February, 1884, p. 127), an article 

 written by the author closed with the following words : "The study 

 of the tetrabranchs teaches us that, when we first meet with relia- 

 ble records of their existence, they are already a highly organized 

 and very varied type, with many genera, and that there was a pro- 

 tozoic period ; and the tetrabranchs, like their successors, certainly 

 must have had ancestors which preceded and generated them in this 

 period, but of which we are at present necessarily ignorant. What- 



