A. H. Foord — Ammonites in the British Museum. 391 



Fig. 12. — Chimcera monstrosa, liinn. : longitudinal section (not quite median) of 

 frontal spine, magnified. The soft cartilaginous (pro-fibrochondral) 

 axis {sa.) is represented by the light shading ; the calcified wall (w.) 

 ■with the blood-vessels {bl.v.) is shaded darker. The black inferior 

 surface of the spine serves for the attachment of a great part of the 

 fibres of the adductor- mandibularis ; l.a.o. marks the attachment of the 

 levator anguli oris. The median canal {m.c), before subdividing into 

 two lateral branches of blood-vessels, has a calcified surrounding wall, 

 separating the soft axis into an anterior [sa-^ and a posterior {sa.) mass. 

 The cutis [cu.) with the denticles (d.) is uncoloured. 



II. — A Short Account of the Ammonites anb their Allies, as 

 Exhibited in the Cephalopod Gallery at the British 

 Museum (Natural History). 



By Arthur H. Foord, F.G.S., 

 Of the Eoyal Dublin Society, Dublin. 



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

 _L 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 

 into which such shells are subdivided. Though typically coiled, 

 much in the manner of the flat pond-snail, Planorbis, 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 the most recent researches in this large and difficult 

 group of fossils: — (1) Arcestid^ ; (2) Tropitid^ ; (3) Cerati- 

 TiD.^ ; (4) Cladisoitid^ ; (5) Pinacooeratid^ ; (6) Phyllocera- 

 TiDiE ; (7) LYTOCERATiD.a: ; (8) Ptychitid^ : (9) Amaltheid^ ; 

 (10) ARiETiDiE ; (11) ^goceratid^ ; (12) Polymorphidje ; 

 (13) Harpoceratid^; (14) Pulchellid^ ; (15) Haploceratid^ ; 

 (16) Stephanoceratid^. 



These sections will be found amply represented in the gallery 

 set apart for their display. 



Omitting certain forms of doubtful relationship, to be subsequently 

 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 of the Alps (Alpine 

 Trias). Among the Arcestid^e may be noticed the singular Triassic 

 genus Arcestes, with its deeply embracing whorls and contracted 

 aperture. In the Ceratitid^ the well-known Muschelkalk species, 

 Ceratites nodosus, with its peculiar suture-line (Fig. 2), and the 

 richly ornamented shell of Trachjceras Aon (Fig. 1), are met with. 



Three very peculiar shells also deserve notice. They are: Choris- 

 ioceras, Cochloceras, and Ehahdoceras. In Choristoceras, the last 

 whorl becomes separated from the preceding ones, in the manner 

 of Crioceras. Cochloceras is turreted like a Gasteropod, and may 



