1895.] HYDBACHNU) FOUND I>' COBNWALL. 177 



not occur on the larger plates. The larger plates are arranged in 

 three irregular longitudinal rows ; the same plate rarely exactly 

 agrees in form or size in two individuals or on opposite sides of the 

 body, and seldom, in form, even on the two sides of the same plate ; 

 but the arrangement may be said to be approximately as follows : — 

 The central line consists of three unpaired plates, of ^which the 

 anterior is much the largest, and much larger than any other plate 

 on the body ; it is heart-shaped. The central and posterior are 

 nearer to a square or oblong form. The lateral rows are each 

 composed of four plates of irregular forms ; the hindmost is nearly 

 triangular and approaches near to its fellow on the other side of 

 the body. I am not sure that in some specimens some of these 

 plates may not coalesce or be broken up into luore than one. 



The smaller plates generally form an irregular longitudinal line 

 of four on each side, starting from between the first large central 

 plate and the eyes and passing between the central and lateral 

 lines of large plates. 



The edije of the body. — There are some of the small plates, and 

 one or two of the nature of the large plates, on the actual lateral 

 and posterior edge of the body. 



The ventral surface (fig. 2). — The epimera are ai'rauged in two 

 groups on each side of the body, those of the iirst and second leg 

 being anchylosed together ; and the same taking place with those of 

 the third and fourth, but a strip of soft cuticle intervening between 

 those of the second and third. Between the epimera of the first 

 pair of legs, but not attached to them, is a plate formed of the fused 

 sternal plate and maxiUary hp ; this plate does not extend as far 

 back as the epimera do. 



The body forms a lateral, almost square, projection between the 

 epimera of the second and third legs. 



There are, on the ventral surface, two pairs of irregular-shaped 

 plates near the posterior margin, of the nature of the larger dorsal 

 plates ; four or five pairs of the nature of the small dorsal plates ; 

 the anal plate (if that be its proper name) ; a small plate just 

 anterior to the anal and like it in form, but turned in the reverse 

 direction ; and, finally, the plates surrounding the genital opening. 

 The anal plate (fig. 10) has a round central opening, which is the 

 exterior orifice of the excretory organ and is closed by two soft 

 labia. This opening is surrounded by a ring of chitin rather 

 dehser than the remainder of the plate, ^\'hich ring is pierced by a 

 regular row of very fine pores. A little further out is a concentric 

 ring of rather larger pores, and the rest of the plate has pores 

 similar to those in the epimera &c. 



The sclerites surrounding the genital aperture are similar in both 

 sexes, and consist of a small, anterior, median, almost triangular 

 plate of rough chitin with the point directed forward, and two 

 ; paired lateral plates, the shape of which will be best gathered from 

 the drawing ; these nearly touch posteriorly, but are further apart 

 anteriorly. Two pairs of the so-called genital suckers are situated 

 • between these lateral plates, and one pair of larger ones behind and 



Peoc. Zool. Soc— 1895, No. XII. 12 



