216 A HISTORY OF RECENT CRUSTACEA 
the third trunk-legs ; and the two teeth on each side of the 
telson in caramote are wanting in canaliculatus. 
Penceus esculentus, Haswell, is said to be the common 
edible prawn of Sydney, Newcastle, and other places in 
Australia, and but few must be required to make a dish, 
if they often reach, as they do sometimes, the length of 
nine inches. It is perhaps not distinct from the type 
species of Fabricius. 
In Penceus and some at least of the other genera in the 
family, there is on the ventral surface of the trunk a struc- 
ture peculiar to the females, to which Spence Bate has given 
the name of Thélijcum (see Plate XII.), ‘which,’ he says, 
‘so far as | am aware, has never been previously figured or 
described by any naturalist.’ Yet he presently after refers 
to one description of it by de Haan, and might have men-- 
tioned that it is described by that author in no less than 
four species. Of the female of Penceus canaliculatus, de 
Haan says that ‘the sternum is channelled between the 
three anterior legs, between the fourth having a narrow 
rounded horny lobe, and between the fifth a broader 
membranacecus orbiculate lobe, which in advance of the 
middle is cleft and embraces the median spine.’ Referring 
to the same species and sex, Spence Bate says :—‘ On the 
ventral surface in both our specimens, between the poste- 
rior pair of pereiopoda, is a large thelycum, consisting of a 
dichotomous, calcareous capsule, which extends forwards 
as far as the base of the antepenultimate pair of pereiopoda, 
whence project two large, leaf-like, membranous appen- 
dages (Plate XXXII. fig. 49). They appear to be con- 
nected with the internal organs by means of foramina in 
the floor of the capsule, and have no connection with the 
fifth pair of pereiopoda.’ Paulson also, in 1875, figures and 
describes these appendages in various species of Penceus. 
The organ appears to vary considerably in different species, 
but the question is complicated by the probability that it 
may undergo important changes of form at different stages 
of the animal's existence. Spence Bate’s figures and de- 
scriptions will no doubt lead the way to the clearing up of 
this question. 
