4 Dr. Mac Cullocli on the Means 



to be folded under the body. Some short muscles are appointed 

 to this office ; but it is unnecessary to enter into further particu- 

 lars ; as enough is given in this sketch of the motions of the limb 

 and the position of the muscles, to illustrate the object of this 

 paper. The beauty of the mechanism, by which strength of arti- 

 culation is combined with extent and facility of motion, cannot 

 fail to strike the most negligent observer of nature. 



It is in the fifth phalanx that the provision for detaching the 

 limb is placed, and the accompanying sketches will assist in ren- 

 dering the description intelligible. 



It will be perceived that, on the exterior side of this phalanx, 

 there is a pale ring, transversely drawn round it (Plate I. Fig. 1.) ; 

 and, with a magnifying glass, it is easy to perceive, though much 

 more distinctly in some species than in others, that a very fine 

 line lies in the middle of it, not exactly in one plane, but slightly 

 undulated. (Plate II. Fig. 6.) If the edge of a knife be forced 

 upon this line, it will be found that the phalanx, at the moment it 

 yields, flies asunder with a loud crack. In every part of the shell, 

 the bony matter is deposited in a fibrous manner, transversely to 

 the plate ; but at this particular part, the fibres are peculiarly 

 fine and straight, while the structure is also more brittle or 

 tender and the colour paler, (Plate II. Fig. 8.) The division is, 

 in fact, a natural suture ; nor is it possible to separate the joint 

 in any other place than where this exists. 



If now a longitudinal section of the whole phalanx be formed, the 

 appearance represented in Fig. 7 will be seen. It will be observed, 

 that the suture occupies a thinner part of the shell, or that this 

 portion is more slender than the general shell of the limb, and 

 that it is bounded on each side by two reinforced rings. This is 

 the case at least with the spider crab ; but it is less visible in 

 some other species which I have examined, although the peculiar 

 structure of the suture, in other respects, is always to be dis- 

 tinguished. I must nevertheless remark, that in some, as in the 

 lobster and crawfish, the external ring is by no means conspicu- 

 ous, though the same provision exists in the internal arrangement, 

 nor is it very obvious, externally, in the cancer pagurus. In the 



