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 lowest 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 
