25 



February 19, 1851. 

 The President in the Chair. 



Mr. Ay res continued his notices of Holothuridce. 



PsoLUS L^viGATUS Ayres. This species is of a very differ- 

 ent type from the Synapta described at the last meeting of the 

 Society. It belongs to the division of Holothurians which have 

 the greater part of the surface covered with strong, imbricated, 

 calcareous scales or plates. Of this division, but two species 

 besides the present have yet been detected on our coast. The 

 suckers, by means of which the animal creeps, are confined to 

 a quadrangular space on the inferior surface. This space has 

 the skin nearly smooth, though strengthened by calcareous 

 deposits. The suckers are in three rows, two lateral and one 

 medial, the rows being joined at their extremities by additional 

 suckers irregularly situated. A European species, allied to this, 

 is stated by Prof. Forbes to adhere so strongly by means of its 

 suckers that in one instance the head was torn from the body by 

 the dredge without loosing their hold. The other parts of the 

 body, except a narrow space below the tentacula, are covered 

 with scales. These are somewhat analogous to the scales of 

 fishes in external appearances but not at all in structure or in 

 relations. A remarkable peculiarity of the calcareous plates 

 occurring in various parts of the structure of many Holothuridse 

 is, that they are perforated with numerous holes, thus causing 

 them to appear, except on close examination, as if composed of 

 minute grains cemented together. This is shown perfectly in 

 the scales of Psolus. When subjected to the action of an acid 

 their intimate structure is readily distinguished, and they are at 

 once recognized as the analogues of the delicate plates of 

 Synapta, and of those which support the tentacula and suckers 

 in most genera. They are not arranged in any regular order, 

 neither are they of uniform size. A few round granules of 

 organization similar to their own are scattered upon them. The 

 fact that these granules are much less numerous than in P. 

 phaiitapus, which this species seems to represent on this side of 

 the Atlantic, has suggested the specific name lavigalus. 



