508 STUDIES ON AUSTRALIAN MOLLUSCA, 



parasites proper and mere commensals. Reckoning Bathyscia- 

 dium, Conchiolepis, Ovula, Thyca, &c, among the latter, I can 

 only find the following described as true parasites : — Stilifer, 

 Eulima, Styliferina, Entoconcha, Entocolax and Robidardia. 



Megalatractus aruanus, Linn. 

 (Plate xxv., fig. 18.) 



In my note on this species {ante, p. 99) I remarked that though 

 certain writers had seen the nidamental capsules of this mollusc 

 they had refrained from giving any information about them. 



Dr. T. H. May, writing from Bundaberg, Queensland, tells me 

 that he sometimes finds these egg-capsules there, occasionally 

 with embryo shells in situ. He most kindly forwarded me a dry 

 and empty capsule which I now figure and describe. 



The mass before me weighs three-quarters of an ounce, measures 

 — length 5, breadth 2|, and thickness 1| inches. The shape is 

 oblong-reniform; the concave side appears to have been attached 

 to some foreign body which passed through an orifice at one end. 

 The capsule is transversely divided into a dozen compartments, 

 hinged together at the back, that is the attached, concave side. 

 Each compartment, now empty, may be supposed to have con- 

 tained several embryos; each fits into its predecessor and receives 

 its successor cup-wise. The outer face of each compartment is 

 puckered into seven or eight keels, producing a series of imbri- 

 cating crests which traverse the mass longitudinally. The keels 

 may indicate the number of cells in a compartment. 



The plan of construction of this ovisac somewhat corresponds 

 to that of Pirula canalicidata, figured by Fischer.* We know 

 too little of the oviposition of the Prosobranchiata to suggest 

 what bearings this may have on the systematic position of 

 Megalatractus. It is obvious from the bulk of the larval shell 

 that M. aruanus can have no free-swimming stage. 



* Manuel, p. 92, f. 85. 



