HETEROCARPUS VICARIUS. 



149 



of the orbit, on a level with the base of the eye-stalk, runs backward and 

 a little downward along about two fifths the length of the carapace, then 



each side of the carapace. The uppermost of these, lettered a iu the accompauyiug cuts, begius at the pos- 

 terior boundary of the orbit, just behind the base of the eye-stalk, ruus backward dedniug the lower Umit of 

 the gastric aud cardiac regions, and ends near the hind margin of the carapace just at the point where the 

 hind margin curves backward to form the hind margin of the brauchiostegite. The middle lateral carina {b) 



Fig. 1. 



Fig. 4. 



Fig. 2. 



Fig. 5. 



Fig. 3. 



Fig. 6. 



originates anteriorly in the orbital spine and courses backward over the hepatic and branchial regions, below 

 carina a. The lowermost lateral carina (c) is continuous in front with the lower spine of the autero-lateral 

 border (the brauchiostegal spine of Stimpsou), and sweeps backward below b along the branchial area uutil it 

 is lost near the hind border of the carapace. Tliis complete expression of all the carina; is represented iu the 

 first diagram (Fig. 1). It is realized iu U. carinalus Smith (^ U. ensifer, juv. ?), iu which all the carina; are 

 developed throughout their whole course, although a is not so sharply defined on the anterior half of the cara- 

 pace as it is posteriorly. Still it can be followed quite up to the margiu of the orbit. The next diagram 

 (Fig. 2) shows the complete suppression of the anterior half of a. This condition is exemplified in H. eiisifer 

 A. M. Edw. Iu the majority of the known species, however, a is perfectly developed, while b is well-nigh 

 completely obsolete (Fig. 3). We then have two lateral carinse. These are not homologous with the two 

 complete carina; of U. ensifer, but with the upper half-cariiia and tlie lowest one of that species. A slight ves- 

 tige of the anterior eud of b appears in the form of a short ridge continued backward from the base of the 

 orbital spiue. In this category come the following species : H. vicarius Fax., gibbosus Bate, oryx A. M. 

 Edw., doTsalis Bate, alphomi Bate, hostilis Fax., and affinis Fax. The next stage in the suppression of the 

 carinse is shown in Fig. 4, where, besides the absence of b, we note the obsolescence of the posterior fourth 

 of c (H. Isvigatus Bate). In H. ahxandri A. M. Edw., c has completely disappeared, leaving only one lateral 

 carina, a (Fig. 5). Finally, in H. Itevis A. M. Edw., all the carinsB are obsolete (Fig. 6), and we have a 

 smooth carapace, as in Pandalus. 



The homology of these carinse has been misapprehended by some writers. A. Milne Edwards, for in- 

 stance, says that the superior lateral carina of H. ensifer is absent iu H. oryx, whereas it is iu fact the middle 

 one that is obsolete in the latter species. 



