234 THE COMPARATIVE MORPHOLOGY OF THE CRAYFISH. 



posterior end of the ridge passes into a somewhat broader 

 and less marked elevation, the hinder end of which turns 

 inwards, and then comes to an end at a point midway 

 between the orbit and the cervical groove. Generally 

 this hinder elevation appears like a mere continuation 

 of the post-orbital ridge ; but, sometimes, the two are 

 separated by a distinct depression. I have never seen 

 any prominent spine upon the posterior elevation, though 

 it is sometimes minutely spinulose. The post-orbital 

 ridges of each side, viewed together, give rise to a cha- 

 racteristic lyrate mark upon the cephalic region of the 

 carapace. 



A faintly marked, curved, linear depression runs from 

 the hinder end of the post-orbital ridge, at first directly 

 downwards, and then curves backwards to the cervical 

 groove. It corresponds with the anterior and inferior 

 boundary of the attachment of the adductor muscle of 

 the mandible. 



Below the level of this, and immediately behind the 

 cervical groove, there are usually three spines, arranged 

 in a series, which follow the cervical groove. The points 

 of all are directed obliquely forwards, and the low€st is 

 the largest. Sometimes there is only one prominent 

 spine, with one or two very small ones ; sometimes there 

 are as many as five of these cervical spines. 



The cardiac region is marked out by two grooves which 

 run backwards from the cervical groove (fig. 61, A, c), and 

 terminate at a considerable distance from the posterior 



