Victorian Fossils, Part XVI. 103 



all intents and purposes comparable to a colloid substance, and took 

 an instantaneous impression of the intricate structural features of 

 the organism before they became deteriorated l)y agents of decay. 



Nofe on a hrachial rdlrr of I.('pt<itii(t r/ioni/ioidalis showing well- 

 preserved impressions of the hrac/iial spiralia. 

 (Plate X., tigs. 4, 5.) 



This shell is in the early neanic stage, and measures only 5 mm. 

 in length by 7.5 mm. in width. It represents the interior of a 

 brachial valve with the fleshy spiralia strongly impresesd on the 

 surface. The positive form of the spiralia is seen in the wax impres- 

 sion. It shows them to have been apparently diplospiral,! but of 

 few convolutions, the apices directed to the front (ventrally) and 

 slightly outwards. That this specimen is the interior of a brachial 

 valve and not merely the impi'ession in mudstone is evident from 

 the fact that true shell structure is present by the distinct perforations 

 all over ithe internal surface. The cardinal area is very distinctly 

 crenulate, more strongly so than in the neanic form herewith figured 

 to illustrate this new character in Leptaena (Fig. 3). These points 

 tend to support the suggestion here brought forward that the 

 denticulate hinge character disappears with age and shell-develop- 

 ment, owing perhaps to the greater strength acquired in later stages 

 by the formation of the dental lamellae adjacent. 



The form of the spiralia, so clearly demonstrated by a wax 

 impression, is even more definite than in the beautiful example of 

 the Gothland specimen figured by Davidson. 2 Moreover, in our 

 example the spires are seen to l)e attached directly to the crura at 

 the posterior end of the septum. 



Note on a brachial valve of Lepfaena rhomhoidalis shovinr/ brachial 

 markings or sujiporfs. (Plate X.. figs. 6, 7.) 



This specimen, discovered by Mr. Sweet, is also, as in the former 

 example, in the neanic stage. It is a very faithful mud-cast of the 

 interior of the brachial valve. The crenulation of the cardinal 

 area is very distinct. The septum is well developed, extending 

 nearly the whole length of the sliell. The bases of the brachial sup- 

 ports follow the general trend of the spiralia, but show a curious 

 angularity of outline towards the anterior margin ; a character 



1 A double spire formed liy the continuity of the bifurcated juguni with the convolutions of 

 the spire. In this particular case the contiiuiation of the jufjal processes was probably of a 

 chitinous nature. • 



•2 Op. cit., pi. xxxix., fijr. 16. 



