C. F. ROUSSELET ON LITTLE-KNOWN PTERODIXA. 25 



Dr. Hudson, reads as follows : " lorica entire, greatly depressed, 

 of two oval or nearly circular plates soldered together at their 

 edges." This last character will have to be modified, as quite a 

 number of the species, though generally depressed, have a lorica 

 of considerable thickness and rounded at the edges, thus giving 

 much more room for the internal organs, and consisting of 

 stiffened integument not separable into two plates, but obviously 

 continuous all round. It is my opinion that even in the very 

 fiat Pt. 2^citi7ia there are not really two plates soldered at the 

 edges, but that it has only this appearance, and that in realit}" 

 the lorica is continuous all round, but closely appressed at the 

 periphery. 



I may here also record my belief that Pt. valvata is only the 

 young form of Pt. 2}(itina. When young the integument is yet 

 soft and fiexible, and the strong retractor muscles of the head 

 being fixed near the edge of the lorica on each side, fold down the 

 sides like flaps when contraction takes place. I have seen this 

 form repeatedly, always in company with Pt. patina^ from which 

 it can otherwise scarcely be distinguished. 



Pterodina rejlexa (Plate 4, Fig. 5). I have met with much 

 diflSculty in identifying this species from Mr. Gosse's scanty 

 description and peculiar figure, but was able to do so after seeing 

 some of Mr. Gosse's original drawings, which have not been 

 published. I received this small animal from Mr. John Hood, of 

 Dundee, in January 1896. From a purely dorsal or ventral 

 view, and with a monocular microscope, the peculiar shape of the 

 lorica can hardly be seen; and it is necessary to observe the 

 animal in the act of swimming, turning round and over many 

 times under the binocular, in order to realise all its peculiarities. 

 The shape of the carapace is oval, obscurely pointed posteriorly, 

 the two sides curved upwards like an open V at an angle 

 of about 130°, and with thickened and rounded edges, as will 

 best be seen by the transverse section, fig. 5c. The anterior 

 frontal margin is raised, and has a very slight depression in 

 the middle, while the mental edge has a very deep and square 

 sulcus. 



The foot opening is situated near the posterior margin of the 

 lorica, and is oval in shape. The lateral antennse are seen 

 protruding about the middle of the lorica, close to the edge. Mr. 

 Ternetz's new species Pt. incisa appears to have considerable 



