32 Guide to the Invertehrata. 



GALLEEY narrow, and the inner whorls are more or less completely concealed 



"^ ' by the outer whorl. They are usually smooth or with only fine lines 



,^Q ' oi growth, rarely with tubercles or ribs. The suture-line may be 



j^ simply waved [Agoniatites) or angulated {Brancoceras) (Fig. 55), 



71, or still more complicated {Beloceras, Prolecanites) (Fig. 56), but 



it is never foliated and incised as in the Ammonites. The 



siphuncle is always small and close to the periphery of the shell. 



Numerous forms of Goniatites are found in the Devonian and 



Carboniferous, and they also occur in the Permo-Carboniferous 



rocks of the Salt Eange, India. 



The great group of the Ammonites (using that term in its 

 general acceptation) is distinguished from all other kinds of 

 chambered shells of the Cephalopod type by the complicated 

 foliations of the margins [sutures) of the partition walls or septa 

 by which such shells are subdivided. Though typically coiled, 

 much in the manner of the flat pond-snail, Planorhis, there are 

 straight and variously curved Ammonites; but all have the 

 common character of a highly foliated ''suture-line." 



The derivation of the Ammonites from the Goniatites has been 

 clearly made out in certain groups by means of this suture-line, 

 the development of which from its earliest stages of growth has 

 furnished the key in such investigations. 



The following sixteen families or sections of the Ammonites 

 are the result of recent researches in this large and difficult 

 group of fossils: (1) Aecestid^ ; (2) Tropitid^ ; (3) Ceeati- 

 TiD^ ; (4) Cladiscitid^ ; (5) Pinacoceratid.i; ; (6) Phyllo- 

 CEEATiD^; (7) LYTOCEEATiDiE ; (8) Ptychitii)^ ; (9) Amaltheid^ ; 

 (10) ARiETiDiE ; (11) ^Egoceeatid^ ; (12) Polymoephid^e '; 

 1 (13) Haepoceratid^ ; (14) Pulchellid^; (15) Haploceeatid^ ; 



(16) Stephanoceeatidje. 

 1 These sections will be found amply represented in the Gallery 



of Cephalopoda, IS'o. VII. 



J Omitting certain forms of doubtful relationship, to be sub- 



^ sequently dealt with, a brief account of the above groups may 



here be given. The first four families are found chiefly in the 



Permian rocks of Sicily and of India, and in the Triassic rocks 



Table-case of the Alps (Alpine Trias). Among the Aecestid^ may be noticed 



6^* the singular Triassic genus Arcestes, with its deeply embracing 



. whorls and contracted aperture ; and among the Teopitid^ the 



! characteristic genus Tropites. In the Ceeatitid^ the well-known 



