376 ME. W. F. LANCHESTER ON THE [DeC. 2, 



The outer maxilla has a deep incision in its edge, at the bottom 

 of which is a spine, externally to it three spines, and internally 

 seven spines ; the inner maxilla is ovoid, with long hairs on its 

 outer surface. 



The peduncle is equal to, or sometimes a little shorter than, 

 the capitulum in length. 



Yariations. 

 This species is very variable in external appearance, one or two 

 of the specimens appearing at first sight to be specifically distinct ; 

 but I have no doubt, from a comparison of all the specimens here 

 present, that they are all of one species. These variations are 

 connected with differences in the structure of the membrane, and 

 the form and shape of the valves. 



(i) The Meinhrane. 



The membrane varies, both in the peduncle and capitidum, 

 from the one extreme, in which it is thin, whitish, and trans- 

 lucent, to the other, in which it is thick, brown (in formol), and 

 opaque, with the valves almost invisible ; transitional stages 

 connect these two extremes. 



(ii) The Valves. 



The tergum is generally shaped like the head and neck of a 

 horse, as described, and formed of a single piece, but varies to 

 the extreme shown in fig. 7 b. In two young forms also, with 

 transparent membrane, the head proper is reduced and the neck 

 thickened, giving an appearance as in fig. 7 c. In one specimen 

 the base of the neck is formed of a very small separate double 

 piece. 



The scutum is generally as described ; but the basal segment, 

 generally as high as wide, is sometimes less developed, and is then 

 much wider than it is high. The basal segment is also very 

 variable in the number of pieces of which it is formed, thus : — 



Individual specimens : basal segment of scutum : 

 {a) Yery small, calcified separate portions are present — one at 



the baso-lateral, and two others at the tergal, angles. 



(&) Formed of two pieces — a small umbonal, and a large distal 



(tergal). 



(c) Formed of three distinct pieces — (1) a relatively small piece 

 round the umbo of the valve ; (2) a large piece, including the 

 basal, and a little more than half of the tei"go-lateral, margins ; 

 (3) a piece, intermediate in size, including the rest of the tergo- 

 latei'al, and the whole of the occludent, margins. The line of the 

 tergo-lateral margin is markedly irregular. 



(d) On the right side (in relation to the animal) of three pieces 

 as in c ; on the left side of two pieces, a large umbonal and 

 somewhat smaller distal (tergal), the suture between them rvinning 

 irregularly from the middle of the tergo-lateral, to the middle of 

 the occludent, margins. The carina is constantly formed of two 



