47 
lIl.-rOM AND STRUCTUIIE OF THE GKNUS EURYDESMA.* 
In dealing with the general characters of the genus, it must he understood 
that, according to our interpretation, there occur in Australia two species 
of Eurydesma, passing through well-marked gradations and varieties — E. 
cor datum, Morris, and E. hohorteuse, Johnston. E. glohosum, Dana, E. 
sacaiUum, E. ellipticum, Dana, and E. antarctlcum, Freeh, sp., are considered 
as appropriately grouped with E. cordatum. The Indian forms described by 
Waagen do not belong to our species, nor is it quite certain that they are 
sjiecies of Enrydesma, Morris. 
For the purpose of convenience, it is projiosed to grouj:) the descriptions 
of the structure under headings. 
1. Form and Contour. — The contour entirely depends on the position 
in which the shell is placed. If oriented in accordance with the Aviculid 
affinities suggested, i.e., with the posterior dorsal margin horizontal, the 
outline is either obliquely cordate {E. cordatum) or obliquely oval {E. 
hobar tense). 
The shell is circular, transversely oval, suhorhicular, or irregularly 
elliptical, sometimes even becoming somewhat cordate. The suhorhicular 
outline is seen in Dana’s figure of E. cordata, Morris,^ the elliptical in his 
E. elliptica,^ and the cordate outline is more particularly aj^proached in 
Sowerhy’s figure of E. cordata, Morris.^ Dana’s E. globosa^ is even more 
cordate, while Waagen describes his E. ellipticum as transversely oval,® and 
E. cordatum as “nearly circular.”® 
Eurydesma, attained a considerable size, and was a heavy shell ; one 
individual of E. cordatum measures six and tlirhe-quarter inches from the 
uml )0 to the ventral margin, and five and a half inches across the valves. E. 
Hobart ense never attains these proportions. 
*Note. — In this section we attach to the various forms of Eurydevma the specific names as applied by 
their authors ; our interpretation will be found later. 
> Dana in Wilkes— PI. VIII, fig. 1. 
" Dana — Op. cit., PI. VII, figs. G a and b. 
* Sowerby in Mitchell — Op. cit., PI. II, fig. 1. 
* Dana in Wilkes — Op. cit., PI. VII., fig. 7. 
* Waagen — Salt Range Fossils, IV, Pts. 5-8, 1871, p. 141. 
® Waagen — Loc. cit., p. 142, 
c 
