REPORT ON THE CRUSTACEA MACRURA. 221 



the structure of the tissue soft and membranous, hut in others, where the structure is 

 hard and rigid, the rostrum is long and slender, as in most of the species of Aristeus. 

 The teeth on the rostrum, whether on the upper or the lower surface, exhibit a tendency 

 to be constant, even in those genera where there is a considerable variation in the. 

 structure of more important organs. 



In Penams the teeth are generally numerous, but vary from seven or eight to twice the 

 number, and in some species they appear on the lower margin, but more commonly they 

 are absent from that position. In Aristeus three teeth are the almost constant armature 

 of the upper surface of the rostrum, and we know of only one species that departs from 

 this character. In Aristeus rostridentatus a number of small teeth arm the rostrum to 

 the apex. In Benthesicymus, where the rostrum is very short, the crest is elevated, and 

 most species carry two small teeth, but in Gennadas there is only one. Thus the number 

 and arrangement of the teeth on the rostrum of the Penasidea may be considered as 

 sufficiently constant and important to be accepted as a ready and convenient guide to the 

 determination and classification of species. 



In Penieus there is constantly a small tooth situated at the anterior extremity of the 

 hepatic region, just behind the furrow that is formed by the remains of the cervical fossa. 

 This tooth is absent in Aristeus in all species except Aristeus rostriclentatus. It is 

 present in Benthesicymus, and absent in Gennadas. In Sicyonia it appears as a formid- 

 able armature, and in Solenocera and its near allies, not only is it present, but there 

 are others which are post-orbital and post-antennal, that appear to be constant and 

 determining features. The frontal margin slightly recedes and has no distinct orbit, 

 but a small tooth that overhangs the base of the first pair of antennas defines the limit, 

 whence the frontal margin recedes still more obliquely, and passes behind the base of the 

 second pair of antennas. Here is frequently situated a strong tooth that forms the 

 anterior extremity of a strong ridge that runs backwards, and meets, without uniting 

 with, another ridge that defines the bmit between the cardiac and branchial regions. 

 The frontal margin still recedes posteriorly until, at a short distance below, it forms 

 another ridge that longitudinally traverses the branchial region to the posterior extremity 

 of the carapace. This perhaps is the more constant ridge, and it is best seen in Aristeus, 

 but it is reduced to a minimum in Benthesicymus. Although the structure of the 

 carapace may be firm and rigid, as in Aristeus, the portion below the ridge is soft, 

 flexible, and membranous ; to such an extent does this exist in some species, as in 

 Hemipenaeus semidentatus, that the vascular ramifications may be seen in the tissues 

 beneath. 



In some species of Penseus very peculiar fissures may be seen traversing the carapace, 

 one in a longitudinal direction from the orbital margin, in a slightly waved line to near 

 the posterior margin, the other vertically, commencing near the centre of the infra- 

 lateral margin, and passing up halfway through the branchial region. It cannot be taken 



