REPTILIAN AGE. JURASSIC PERIOD. 117 



or provided with lateral appendages. Septa more t)r less deeply lobed 

 on the margins ; presenting a convex outline (in their mesial section) on 

 the side facing the aperture ; lobes variously plicated or sinuous . and 

 dentate or merely serrated on the margins. Siphon dorsal, with relation 

 to the shell, cylindrical, slender, never occupied by an internal organic 

 deposit, piercing the septa from within outwards, or towards the aper- 

 ture ; envelop solid and persistent. 



Animal unknown, all the genera of the entire group being extinct. 



This family is nearly related to the Goniatitidce, through the intermediate Ceratites. 

 Indeed, previous to the researches of the distinguished palseontologist Barrande, the 

 Goniatites were by most authors, along with the Ceratites, included in the family 

 Ammonitidce ; and some even included these three genera under the single generic 

 name Ammonites. M. Barrande, however, has shown (Bui. Geol. Soc. Fr. 2, 

 ser. t. xiii, p. 375, 1856) that the Goniatite group differs from the true Ammo- 

 nitidce, not only in the greater simplicity of their septa, but also in having the 

 neck or gullet of the siphon always projectmg backwards, as in the Nautilidce, 

 instead of forwards, or towards the aperture, as in the Ammonitidce. Again, he 

 finds that a mesial section of their septa shows a concave, instead of a convex, 

 outline on the side facing the aperture ; while their siphonal envelop is not solid 

 and persistent, as is usually the case with Ammonitidce. 



From these facts some authors have gone to the opposite extreme, and included 

 the Goniatite group in the Nautilidce, M. Barrande, however, has shown, in the 

 paper above cited, that at the same time that they agree with the latter family in 

 these several characters, they still differ in some important elements of structure. 

 In the first place, they always differ in having the septa provided with a dorsal lobe, 

 and generally in having their septa more lobed or sinuous on the sides. Another 

 important difference is the entire absence of the peculiar organic deposit within the 

 siphon, such as we sometimes see in extinct forms of the Nautilidce. Again, they 

 differ in having the siphon invariably on the outer side, instead of varying in its 

 position between the dorsal and ventral margins. Hence we are inclined to agree 

 with.M. Barrande in separating these cephalopods into the three distinct families, 

 Nautilidce, Goniatitidce, and Ammonitidce. 



It is an interesting fact that, even after excluding the Goniatite group from the 

 family Ammonitidce, Ave still have a very extensive and varied group of shells, 

 amongst which we observe a representative, so far as form is concerned, of nearly 

 every genus, not only of the Nautilidce, but also of the apparently distinct Ortho- 

 ceras group. This fact would seem to argue, either that the Nautilus and Ortho- 

 ceras groups should not be separated, or that there may be genera belonging to 

 more than one family included in the Ammonitidce, even as here defined. Still, 

 notwithstanding the great differences of form observed amongst these fossils, they 

 agree so very nearly in their internal structure, that it seems difficult, in the present 

 state of our knowledge, to point out constant characters by which they can be 

 divided into distinct families, or even well-defined subfamilies. 



When we take into consideration the infinite diversity of beautiful forms pre- 



