The Lineage of Tragophylloceras loscombt. S25 
at Dalby are constricted specimens ranging up to 13 mms. in 
diameter. It has been pointed out by B. Smith* that the 
heaps at Dalby probably contain representatives of all the zones 
from oxynotus to jamesoni, and it is therefore*to be regretted 
that the exact stratigraphical relationships of T. loscombi and 
its constricted variety cannot be satisfactorily studied. Light is 
thrown on its phylogenetic position, however, by a consideration 
of the development of a series of specimens representing the 
intermediate stages between the most primitive form found 
(fig. la, b,) and the normal 7. loscomb:. 
Details of such a series are given below. The more advanced 
members are characterised by a greater degree of involution 
and by a smaller number of constrictions, which are confined 
to earlier stages in shell growth. 
Umbilicus Number Last 
Diameter. % (at diam. of Constriction 
75 mms.) Constrictions. at 
Type 4 (typical 
iloscomiben. (LO 22. ZO Ree Me Samim) 
PER Oo MnEn Seen elss theses Dt ae BG ee al Teo Oem nis. 
we Ya oii a ee PShiee ao. so lke eee Sellkannitly, 
constricted. 
ela (Figo e Soa Poe 24 8b) 05,0 9? Meche 
constrictions. 
It appears therefore that we are here dealing with successive 
stages of a lineage or genetic series, the characteristic features 
appearing at earlier stages in the more advanced members 
owing to acceleration in development. 
This conclusion is further supported by a comparison of the 
shells and sutures of the two most widely separated forms. 
The sutural development of the specimens of Tvagophylloceras 
loscombt (Type 4) examined does not differ materially from 
that figured by Spath; ¢ the development of the suture in the 
constricted form (Type 1) is given in fig. 2 a-f. It will be 
noticed that the first suture (fig. 2a) is of the usual angustisellate 
character ; the “incipient ventral lobe’’ observed by Spath 
in some examples of 7. loscombi is not present. Much greater 
differences are shown in the second suture. The shallow 
ventral lobe, without a median saddle, is suggestive more of 
Phylloceras heterophyllum § than of T. loscombi. The third suture 
(fig. 2 c) is similar to the second suture of the typical /oscombi 
figured by Spath, so that here again the acceleration in devel- 
opment revealed by the constrictions, is quite evident. By the 

* Mem. Geol. Survey, Sheet 142, ‘Geol. of Melton Mowbray, &c.’ 
1909, p. 36. 
mw iCocecit. Pigal p34): 
§ Branco, W. ‘ Beit. Entwick. foss. Ceph. ’ ‘Th. I., Pl. IX., fig. 1, 1879. 
1916 July 1. 
