340 MB. L. F. SPATH OX THE [Dec. 1914, 



view ' from above.' Figs. 1 b, 2 b, and 3 b could similarly be styled 

 'aspect from below.' There is very little variation in shape, not 

 only in these three but also in all the other initial chambers 

 examined; this agrees with what Branco says about their constancy 

 in a given species. As that author has pointed out, it is probable 

 that the embryonic shell is represented by only a part of this 

 initial chamber, and the use of the term embryo in Quenstedt's 

 sense for young ammonites up to various diameters is to be 

 deprecated. 



The initial chamber contains the commencement of the siphuncle ; 

 but no sign was discovered of the structure (prosiphon) observed 

 by Munier-Chalmas in the first chamber of ammonites, which 

 Hyatt compares with the siphuncle in Endoceras. The mode of 

 preservation (casts in iron-pyrites, which easily break up into the 

 component chambers on treatment with dilute acid, the interstices 

 being filled with calcite) may be unsuitable for the survival of so 

 delicate a structure ; but it must be remembered that neither could 

 Branco find any prosiphon such as that described by Munier- 

 Chalmas, although Hyatt did so in the bulb of Spirilla {teste 

 Blake, 1892, p. 291). 



The first septum, separating the protoconch from the succeeding 

 (first actual) chamber, is an oval with a long axis like that of the 

 protoconch, transverse to the plane of coiling (see PI. XLVIII, 

 fig. 2 c). It is pierced ventrally by the large siphuncular tube. The 

 curvature of the first suture-line out of the plane of the oval 

 consists of a large ventral saddle : that is, the ammonite is angusti- 

 sellate (PI. XLYIII, fig. 2 a). A minute indentation at the 

 summit of this ventral saddle (incipient ventral lobe) was observed 

 in several cases (PI. XLYIII, fig. 5, and text-fig. 1 a). Branco 

 (1879) had noticed this indentation in only a very few ammonites. 

 The protoconch of Tr. loscombi differs somewhat in shape from that 

 of JPhylloceras lieteropliyllum as figured by Branco (1879, pi. ix. 

 fig. 1), and approaches perhaps more to that of MonopJiyllites 

 [Lytoceras] simonyi (1879, pi. viii, fig. 5). All these initial 

 chambers are characterized by their comparatively large size, 

 whereas that of JPsiloceras planorbis, according to Branco (1879, 

 pi. x, fig. 3), is decidedly small. However, as that author has 

 already pointed out, this is an unimportant character, and probably 

 connected with local (environmental) conditions of development. 



The second suture already has a median lobe (fig. 5, PI. XLYIII, 

 and text-fig. lb) ; and the development of the following sutures, up 

 to a diameter of about 20 mm. where there are six saddles and lobes 

 (delineated in text-fig. 1, p. 31-1), will have to be traced in detail later. 

 The section of the shell up to this diameter represents a gradual 

 transition from a depressed to a circular and finally compressed 

 character. A comparison of figs. 4 a & 6 of PI. XLYIII with 

 figs. 2 b, 3 a, & 4 b of PI. XLIX will show this clearly. The first 

 three figures, taken at the second, seventh, and twentieth suture's 

 respectively, represent speeimens from 10 feet above the Belemnite- 

 Stone : that is. 3 feet above the 'Lower Limestone' on Black Yen. 



