152 Dr. W. T. Caiman on the 



cutting-edge, and is followed by a row of serrated spines 

 extending towards the molar process. In the Euphausiidae 

 and Decapoda no lacinia mohilis is found in the adult, though 

 in the larvai of both a group of serrated spines is sometimes 

 present, which disappears in the course of development. 

 Even in the adults of some of the more primitive Decapods, 

 for instance in certain Atyidge *, a tuft or row of stout bristles 

 is found just below the cutting-edge, and it seems probable 

 that this is a vestige of the spine-row of the Peracaridan 

 mandible. 



In distinguishing the Peracaiida from the Eucarida, 

 Hansen attaches great importance to certain characters pre- 

 sented by the thoracic limbs. Boas had already pointed out 

 that the Mysidas and the Edriophthalmate orders have these 

 limbs terminated by a claw-like spine, which is absent in the 

 Euphausiacea and Decapoda. Hansen regards this claw as 

 representing a segment of the limb, and identifies it with the 

 minute terminal segment which he has discovered in the 

 Leptostraca. Boas had further indicated a difference between 

 the two groups in the direction of the articulations of the 

 limbs. In the Peracarida the *'knee" or chief ventral 

 flexure of the leg is between the fifth and sixth segments, 

 counting from the base, while in the Eucarida it is between 

 the fourth and fifth. Hansen interprets this difference in the 

 following manner : he assumes that the position of the knee 

 is the same in both cases, that the apparent fourth segment 

 of the leg in Eucarida is equivalent to tlie fourth plus the 

 fifth in the Peracarida, and tliat the three segments beyond 

 the knee in the former case are homologous with the two 

 segm.ents and the terminal claw in the latter. If this sug- 

 gestion be correct, we have a difference of a very marked 

 kind between the two groups. Dr. Hansen will doubtless 

 produce further evidence in its support when his researches 

 are published in full, but at present there are difficulties in 

 the way of adopting it as a basis for classification. In 

 certain primitive Isopoda (Janiridae &c.) the leg terminates 

 in two, tometimes three, claws, not differing greatly in size 

 or perceptibly in structure, and it is difficult to believe that 

 one of them is to be regarded as the terminal segment while 

 the others are simply modified setie. Further, in many 



■which are often closely connected with it (" Cirolanidse," Videusk. Selsk. 

 Skr. (6) V. (1890) p. 276, footnote). In the present paper I have used the 

 term in its original and more restricted sense. 



* Cf. Caiman, " On Two Species of Macrurous Crustaceans from Lake 

 Tanganyika," Proc. Zool. Soc. London, 1899, p. 705, pi. sxaIx. fig. o. 



