508 VERRUCID^E. 



tenuity, viz. ^^th of an inch in diameter. The sheet of 

 cement on the whole under side of the basal membrane, not 

 rarely shows a very irregular reticulated structure. For 

 convenience sake, it will be best to defer the discussion on 

 the very anomalous, though slight, powers of excavation which 

 this genus possesses, and which I must attribute to the 

 effects of some substance secreted probably by the cement- 

 organs. I will here only mention, that the specimens which 

 have excavated a depression, are less firmly attached than 

 those, which have not acted on their support; and that, in 

 the former case, the basal membrane, for a considerable 

 space in the middle, becomes quite detached. 



Animal 9 s Body. — The body is much flattened and, owing 

 to the little development of one side of the shell, lies parallel 

 to the surface of attachment. The prosoma is but little 

 protuberant. The articulations of the thorax are unusually 

 straight and transverse. The Mouth is also much flattened: 

 it is placed rather distantly from the adductor scutorum 

 muscle, owing to the production of the lower margin of the 

 labrum. The Labram is not notched, or even hollowed out 

 in the middle, or (excepting in V. nexa) bullate ; its crest is 

 surmounted by about eight (more numerous in V. nexa) little 

 teeth, or by some fine bristles. The Palpi are of moderate 

 size, with their tips nearly meeting ; they are slightly curved, 

 and have bristles only on their outer sides and extremities : 

 they are apparently capable of being lifted up and down by a 

 muscle attached to them, just outside the rounded swelling on 

 each side of the labrum to which they are articulated : in 

 V. nexa, however, the palpi are very small and narrow, and 

 their tips do not nearly meet. In this genus, therefore, we find 

 the swollen state of the labrum and the size of the palpi — 

 characters generally invariable and of high classificatory im- 

 portance — variable. The mandibles have three upper main 

 teeth, with two or three minute lower teeth, or, in V. nexa, 

 with the lower part pectinated with small spines : in V. 

 Stromia, I have seen traces of the second tooth being laterally 

 double — a character of some importance. The Maxilla have 

 a notch under the upper pair of large spines, with the lower 

 part bearing, as usual, a double row of bristles, and forming 

 a large step-formed projection : these organs are furnished 



