550 Instances of Change of Form 



[p. 273.]? and a host of the Porcellana Linneawa [p. 394, 395, 

 552.]. 



This crab differs from the Eurynom^ aspera in the spinous 

 rostrum ; the position of the eyes, which are situated some- 

 thing similarly to those of the male of the E. aspera ; in the 

 sides of the shell having only two lamellae, and those of a 

 different shape, the other appendages being spines ; in the 

 antennae not being so long as the rostrum ; in the elongated 

 shape of the shell ; and, if I am correct in my observation, 

 in the abdomen being tuberculated. That it is not the young 

 of the aspera, may be argued from its shape being farther re- 

 moved from the common type of the Brachyura than is the 

 shape of the known species, that being, in the female, rounded; 

 in this, triangular : but it must not be concealed, that, in a 

 specimen of a short-armed individual, in Dr. Leach's collec- 

 tion, in the British Museum, of about twice the length of the 

 one in question, the three hindmost marginal lamellae are re- 

 presented by blunt teeth ; and, though I could not discover 

 any spines on the rostrum, the largest female in the collection 

 has its external margin with a nsoavy outline, which may denote 

 the former existence of them. 



Notices of some Instances of some Change qf Form in certain 

 cited Species qf Crustaceous Animals. — In fig. 48. is repre- 

 sented the carapace of what I take to be Piliimnus 

 hirtellus in a very young state ; and I send it you 

 because it illustrates very clearly the kind of change, 

 in shape, which some of the Brachyura undergo 

 before they acquire their final form. The full- 

 grown specimens of this crab are transversely elongated, 

 having five spines behind each eye ; and this young one is 

 nearly square, and has the rudiments of the same number 

 of spines. Now, the largest individual I have seen is nearly 

 I in. across, and, consequently, the one in question must be 

 very young; and from this it appears that, at a very early 

 period of their existence, they possess the characters that dis- 

 tinguish the older ones ; while the aberration from the common 

 types of this order is the result of age. In a specimen of 

 the Carcinus Mannas, of the same size as the crab figured 

 above (fig. 47.), I have found this to be the case, the spines 

 being all present, but the form more square than the full- 

 grown specimen. 



But the above observation does not hold good with regard 

 to all the Brachyura, for, in a specimen of the Cancer Pagu- 

 rus, one third of an inch in breadth, which is smaller in com- 

 parison with the full-grown animal than any of the preceding, 



