186 THE VOYAGE OF H.M.S. CHALLENGER. 



pace. On this ridge, on each side of the base of the rostrum, is a very large and acute 

 tooth directed anteriorly and outwards ; behind which on the same ridge are two 

 other sharp teeth that decrease in size posteriorly ; behind the last tooth the ridge fades 

 away. Between the two ridges over the gastric region is a central but less prominent 

 elevation, which commences posterior to the base of the rostrum and terminates a little 

 in advance of the cervical depression. On the frontal margin, beyond the outer angle of the 

 orbit, is a strong sharp tooth, directed anteriorly as far as the base of the scaphocerite ; 

 its outer margin is thick and strong, and extends back as a prominent ridge to the 

 anterior or hepatic sulcus. In the depression between the ridge formed by this large 

 tooth and the dorsal carinas are two small teeth, one immediately behind the orbit, 

 the other obliquely posterior to it, and about half way between the two ridges. On the 

 region bounded by the hepatic sulcus and the cervical groove, somewhat below the line 

 of the great antennal tooth, is a small sharp tooth directed forwards and outwards between 

 which and the dorsal carinse are four or five small teeth standing equidistant from 

 each other on the posterior margin of the hepatic sulcus ; these gradually increase in 

 size as far as the place where the sulcus fades away (PL XXVI. fig. 1). 



The posterior half of the carapace comprises the cardiac and branchial regions 

 and possesses traces of three or four semi-obliterated carinae, each of which is defined 

 anteriorly by a small sharp tooth, and traversed by a series of granulations that are 

 of the same character as those that stud the branchial region, but larger. The median 

 carina is double, and appears to be an imperfect continuation of the small central 

 ridge on the median line of the anterior half of the carapace. It is furnished just behind 

 the cervical sulcus with two anteriorly-directed sharp teeth, and four or five other less 

 important ones in pairs, some of which are sharp, and others like small tubercles, and 

 they continue almost to the posterior margin, which is bordered by a broad, flat, but 

 elevated band that traverses the posterior and infero-lateral margins of the carapace to 

 near the anterior extremity of the branchial region, gradually fading away as it extends 

 forwards. 



The first somite of the pleon is narrow, not reaching beyond, or even quite to, the 

 second lateral carinal ridge of the carapace ; the anterior half, when the animal is 

 extended, underlies the carapace, while the posterior half is slightly more elevated 

 throughout, and overlies the posterior margin of the carapace laterally, producing the 

 pleocleis, which keeps down and secures the carapace in position. 



The second somite is loiiQ-er and broader than the first. The dorsal surface is divided 



O 



into two portions. The anterior, when the animal is completely extended, underlies 

 to half its extent the first somite ; the posterior overlies the anterior portion of the 

 next succeeding somite, and is divided by two narrow and deep sulcis that commence 

 near the median line on each side, and runs obliquely to the postero-lateral angle, leaving 

 a smooth division between them. The coxal plate, although fused with the true somite, 



