Vol. 70.] DEVELOPMENT OF TRAGOPHVLLOCERAS LOSCOMBL 345 



costation, which is more pronounced than usual. One other 

 fragment and a small cast show a distant sigmoidal costation, 

 which, together with the sharpened and very slightly notched 

 periphery, recalls young forms of Tragophylloceras ibex. Specimen 

 Xo. 13 and a small fragment are characterized by a very unusual 

 straight costation, thickening towards the slightly - sharpened 

 periphery. 



A complete mouth-border does not seem yet to have been 

 discovered, although body-chambers of Tr. loscomhi are fairly 

 common. This might be explained by the fragility of the shell. 

 In the case of Tr. ibex, however, a complete mouth-border is by 

 no means rare, although Pompeckj (1893, p. 22) says that it is 

 unknown. The longest bodv-chambers of Tr. loscomhi that were 

 examined measured nearly three-quarters of a whorl. There also 

 is no evidence as to the presence of an aptychus. 



The development of the suture-line will now be traced 

 in detail. 



■ The ventral portion of the first suture has a large external 

 saddle, two lateral lobes, and two small lateral saddles. The ex- 

 ternal saddle resembles that of the first suture of Monophyllites 

 simonyi, as also of JPhylloceras heterophyllum (Branco, 1879, 

 pi. viii, fig. v [Lyfoceras simonyi] and pi. ix, fig. 1), and is less 

 triangular than that of Psiloceras planorbis [^Eyoceras planorbis] 

 (pi. x, fig. 3). The lateral saddles are intermediate in size between 

 those of the two first-mentioned forms. The dorsal portion of 

 the first suture consists of a median shallow lobe, two saddles, 

 and two indistinct lateral lobes (see text-fig. 1 a, p. 341). 



The second suture belongs to what Branco terms the more com- 

 plicated (but far more usual) type, and presents, besides the external 

 lobe, two external saddles, two lateral lobes, two lateral saddles, 

 and already two indistinct second lateral lobes. The second suture 

 generally meets the first one at or near the umbilicus. It will be 

 noticed that the ventral lobe is already divided by a median saddle ; 

 Avhereas in JPhyJIoceras heterophyllum this division does not occur 

 until the third suture, and in Monophyllites simonyi not until the 

 fourth or fifth. 1 



The third suture closely resembles the second ; but, in the 

 subsequent sutures a second lateral saddle is developed at the um- 

 bilical end, and in the seventh suture both the ventral and the lateral 

 lobes have become deep and the saddles very pronounced. The 

 internal (dorsal) lobe has also deepened, and a second saddle 

 appeared at the umbilical end. 



The undivided suture of the 'goniatite-stage ' persists, as in all 

 ammonites, to a diameter of about 2 5 mm. The 'ammonite- 

 stage' is already indicated at that diameter by the presence of a 

 notch on the internal half of the external saddle, and the following 

 suture (text-fig. 1 e, p. 311), taken at a diameter of 3'25 mm., shows 



1 Branco is not quite sure about this (1879, p. 32). • 



