WILLIAMSON, ON SHKLLS OF CRUSTACEA. 41 



the areolae, explaining the apparently intercellular reticula- 

 tions seen both here and in the common crab, as well as in 

 numerous other Crustaceans. The calcified and uncalcified 

 portions of the corium are much more distinct here than in 

 the common crab ; since the former, instead of consisting 

 of continuous layers of equal thickness, appears in the form 

 of large hemispherical masses, the confluent bases of which 

 ai'e directed outwards, whilst their opposite convex surfaces 

 project inwardly into a thick laminated uncalcified corium. 

 The parallel laminae of the corium continue their straight 

 course equally through the calcified and uncalcified portions, 

 demonstrating that the former are not secretions in the latter 

 pushing aside its layer, but irregular calcifications of their 

 substance. Tubular hairs are implanted in superficial de- 

 pressions in the integument, at which points both the 

 pellicular and areolated layers are wanting. The tube within 

 the hair is continued downwards through the entire thickness 

 of the corium, opening at its inner surface. A calcareous 

 cylinder surrounds this canal, even where it passes through 

 portions of corium otherwise uncalcified. 



Similar, but still more strongly marked, conditions occur 

 in Pi/umnus hirtellus, a vertical section of part of the carapace 

 ofwhichisrepresentedinfig.il. a is the pellicular layer, 

 which here (as is also the case in Hyas aranevs) is the seat 

 of the deepest colour, b is the areolar layer, displaying 

 the dome-shaped areolae with remarkable clearness. These 

 domes are obviously portions in which the calcific matter has 

 assumed different conditions to those under which it has 

 been secreted in the superimposed pellicular layer. Each 

 areola displays some very delicate vertical lines.* Whether 

 or not these are tubes I have been equally unable to satisfy 

 myself, here and in Hyas araneus ; but, as I think that in 

 Portumnus depurator there can be no question that the areolae 

 are tubulated, it is the more probable that the structure in 

 question is also tubular. 



Large hemispherical concretions occupy the upper part of 

 the corium, the centre of each one of which is penetrated by 

 a vertical canal, prolonged downwards through the uncalcified 

 corium, but not surrounded in the latter part of its course by 

 a calcareous cylinder, as is the case with Hyas araneus. 

 Superiorly, each of these canals communicates with the base 

 of a large tubulated hair implanted in a depression penetrating 



* Seen to the left of the drawing; from the opposite portion of which 

 they have been omitted, along with similar ones in the corium, to avoid 

 confusing the other characteristic features of the structure. 



