246 CEYLON PEARL OYSTER REPORT. 



valve well developed, longer than the anterior tooth, diverging posteriorly, its upper 

 margin excavated to form the ligamentar fossa. Found only in Heteropsammia. 



The differences in the cardinal teeth between the specimens found in Heterocyathus 

 and those found in Heteropsammia appear to be constant, and are sufficient to 

 justify my ranking them as distinct species. In addition, the mature individuals of 

 J. heterocyathi seem to be rather smaller, the concentric ridges seem to be more 

 prominent and the posterior and ventral margins of the valves appear to be more 

 rounded than in J. li eteropsammice. 



As may be seen from an inspection of figs. 2 and 3, the ligament is distinctly 

 dorsal and exterior to the posterior cardinal tooth of the left valve, and is contained 

 in a fossa lying just behind the umbones. The hinge, therefore, is not that of a 

 Mactra, to which it has a siiperficial resemblance from the characters of the lateral 

 teeth, nor yet that of a Scrobicularia or Syndosmya. It may rather be compared 

 with the hinge of a Lucina or Diplodonta in which the ligament has become very 

 much shortened and enclosed by the overgrowth of the valve margins dorsally and 

 posteriorly. On the other hand, the excavation of the posterior cardinal tooth 

 suggests a first step in the evolution of the spoon-shaped ligamentar tooth of the 

 Myidas and many of the Anatinacea (Tliracia, Anatina), and, as will be seen, the 

 anatomy of Jousseaumia suggests some affinities with the Anatinacea. 



ANATOMY AND HISTOLOGY. 



As Jousseaumia heterocyathi and J. heteropsammice do not differ from one another 

 in any important anatomical feature, the following account will apply to both species. 

 A general view of the anatomy of J. heterocyathi, as seen in optical section, is given 

 in fio-. 1. 



The mantle edge is thickened and muscular, but there are no pallial tentacles, no 

 eyes or pigment spots. There is a single pallial suture (figs. 16, 17, 18, p.s.) 

 separating a rather elongate anal or exhalant orifice from the large pedo-branchial 

 orifice, the latter extending as far forward as the anterior adductor muscle. The 

 mantle edges are somewhat jDi'ominent and the radiating muscle fibres are rather 

 better developed in the region of the anal orifice than elsewhere, but there is no true 

 anal sijjhon. 



The foot is large, more or less linguiform, and geniculate like that of Cardium. 

 There is a specially strong muscular band running down the posterior margin of the 

 foot, below the byssus groove, and the sudden contraction of these fibres would have 

 the effect of straightening the foot and enabling the animal to spring like a Trigonia 

 or a cockle. It is difficult to conjecture of what use the geniculate and muscular foot 

 can be to an animal leading a sedentary existence embedded in the skin of the 

 Aspidosiphon with which it is commensalistic ; but as 1 find it very well developed in 



