204 BRITISH FOSSIL CRUSTACEA. 



the body, 9 inches in length, the heart was 4 inches 8 lines in length. It was included 

 in a delicate membranous sac analogous to a pericardium, but forming, in fact, the wall 

 of a venous sinus. This wall consists of two layers. One may be properly termed a 

 ' tunic ;' it includes extremely delicate fibres, chiefly transverse, but reticularly inter- 

 woven in a fine cellular bed, the inner surface of which has been the seat of formifaction, 

 or vital crystallisation, of particles from the contained fluid, forming an epithelium, 1 

 the whole resembling a fine arachnoid membrane. The heart is fusiform, widest at its 

 hinder third, gradually narrowing, transversely, to its fore end, which is 2^ inches from 

 the fore part of the cephaletron, more rapidly contracting to its hind end, which is 1| 

 inch from the joint of the tail-spine. In the vertical diameter (PL XXXIV, fig. 1, r, r) 

 the hinder contraction is more gradual. 



" The heart-wall consisted of an outer, thin, smooth, compact coat, and a wall of 

 striate muscular fibres consisting of a thin outer longitudinal layer, and a thicker trans- 

 verse or circular series. The wider part of the heart shows traces of an epithelial lining 

 due to the action initiated or invited by a surface in contact with the formifying 

 material in solution. The arachnoid coat of the pericardial sinus is reflected over the 

 outermost proper tunic of the heart, and is continued into the venous ostia where it 

 gains thickness. 



" These ostia are sixteen in number, arranged in seven pairs at the sides, but 

 towards the dorsal surface, of the heart, with a terminal eighth pair. The hinder 

 ' ostia ' are rather nearer together than the others. The muscular tunic in the intervals 

 of the ostia (PL XXXIV, fig. 1 r, r) is about a line in thickness, but thins off rapidly 

 at the two ends of the heart. Each ' ostium ' (ib., o, o) is provided with a pair of 

 narrow semilunar valves, placed with the intermediate slit almost transversely to the 

 axis of the cardiac tube. 



" The foremost artery runs to the ' ocelli,' is there connected with, or seems to enclose, 

 the nerve ; it then bends down, following the curve of the carapace to the angle formed 

 by the upper with the flat under surface of the digging shield, near which angle the 

 artery is reflected backward and cannot be further traced as a distinct tube. On each side 

 of the origin of the ' ocellar ' artery arises one of double the size, which, diverging from its 

 fellow, curves outward and downward over the fore-part of the intestinal canal (PL 

 XXXIV, fig. \,s) ; it gives off, in this course, a branch which ramifies upon the gizzard, 

 a second to the intestine and liver, the main trunk being continued to the nervous annular 

 centre (ib., (5), where it expands, and combines with its fellow of the opposite side to 

 form a sheath for that centre analogous to a ' dura mater.' This rather loose sheath is 

 continued along the ganglionic ventral chord, and is prolonged, like a loose neurilemma, 

 upon the nerves sent off therefrom, as it is upon those in connection with the annular 

 centre. 



" Fine size-injection being thrown into the ' heart ' from behind forward appeared to 

 1 Owen, ' Anat. of Vertebrates,' vol. iii, p. 499. 



