216 A HISTOEY OF KECENT CRUSTACEA 



the thii-d trunk-legs ; and tlie two teeth on each side of the 

 telson in ca/ramote are wanting in cd/naliculutus. 



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 geiiei'a 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 Thelycum (see Plate XII.), ' which,' he says, 

 ' so far as I 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 

 membranaceous 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. 4 $ ). 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 1 875, figures and 

 describes these appendages in various species of Penoem. 

 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. 



