92 CRUSTACEANS OF TTIE 



Subgenus Potamon s. s. 



Potamon {Potamon) mahakkamense, n. sp. 

 (Plate 8, Fig. 8). 



One adult female, without eggs, was captured by Dr. 

 Nieuwenhuis at Bloe-oe, a locality situated ou the Upper 

 Mahakkam, in the source-region of this river, not far from 

 the source-region of the Upper Kapoeas. 



This species bears such a striking resemblance to Pot. 

 sinuatifrons H. M. E., three type-specimens of which from 

 the Paris Natural History Museum are lying before me, 

 that I at first hesitated to regard it as different. The three 

 type-specimens, all males and collected by M. Leguillou 

 during the Voyage of la Zélée, are of different size, the 

 largest, however, is still considerably smaller than the female 

 from the Interior of Borneo. Potamon m^ahakkamense now 

 differs from Pot. sinuatifrons 1° by the extraorbital tooth 

 being slightly longer with regard to the distance between 

 the external orbital angles and having a more oblique 

 direction, 2° by the characteristic granulation 

 with which the whole upper surface of the carapace is 

 covered. Though the specimens of Pot. simiatifrons are 

 varnished, this granulation seems, indeed, to want com- 

 pletely. For the rest, as regards the general shape of 

 cephalothorax and legs, this species almost fully agrees 

 with Pot. sinuatifrons. 



The carapace is rather much enlarged, the proportion 

 of its breadth to the length (the abdomen excluded) being 

 as 4:3, just as in Pot. sinuatifrons ; it is much broader 

 at the anterior branchial regions than posteriorly and so 

 presents a cordiform aspect. The upper surface is flattened 

 posteriorly, moderately convex anteriorly and declivous 

 towards the orbits and the front. The interregional groo- 

 ves are very faint. The semicircular groove is shallow, in 

 Pot. sinuatifrons it is deeper and the oblique lateral por- 

 tions of the cervical groove are also very shallow. They 



Notes from the Leyden Museuin , Vol. XXI. 



