122 Anuhjlical Notices. 



Body contractile, slightly flattened, composed of seventeen 

 rings, terminated by a prehensile disk ; head oblong, famished 

 with two lips ; mouth with two horny, triangular maxillae, the 

 upper one the larger ; without eyes. 



The situation of this genus is between Hirudo and Trochetla. 

 It is Tiviparous. 



Name of the species, B. Astaci, B. 



Memoir on ACHLYSIA, a new genus of Trachean Arach' 

 nides, by M. J. V. Aubouin. 



This singular parasitical animal, discovered by M, Audouin, 

 attached to the upper part of the abdomen of Dytiscus marginalis 

 under the wings, is of an ovoid general form, very much resem- 

 bling a common glass retort, with an elongated bulb, and a very 

 short rounded neck, abruptly recurved upon the bulb, and form- 

 ing a deep notch. Its colour, after having been preserved for 

 Several months in spirits of wine, was a bright yellow orange, 

 Taried with lemon yellow ; its head, if indeed it exists, is not 

 distinguished from the body : it has no eyes, nor antennae, nor 

 thorax, nor is the body divided into segments, — in fact, it appears 

 to possess neither respiratory nor excretory organs : but it has 

 six legs and a small beak (su^oir), behind which is placed the 

 sternum, divided into three pairs of squarish laminae, and may be 

 considered as the first joints of the legs, one of which takes its 

 rise from the upper corner of each of these square lamina; : the 

 legs have six joints each, and are terminated by a point : the beak 

 and legs are placed in the above-mentioned deep notch, so that 

 when attached by its beak, the animal can only lie on one side. 

 M. Audouin places the Achlysia next to Leptus of Latreille, and 

 names the species A. Dytisci, 



Explanation of Plate 4. 



f. 2. Shows two specimens of Achljjsia Dytisci^ of tlieir natural 

 size, adhering to the abdomen of a Dytiscus, whose elytra and 

 wings have been taken away to expose them. 



