Observations on the Onhatidse. By A. D. Michael. 25 



most frequently joined by a transverse ridge of the same nature, 

 which, in some species, as Orihata sefosa, 0. orbicularis, &c., is 

 large and conspicuous, almost as much so as the lamellae themselves ; 

 in most species, however, the cross ridge is much smaller, often a 

 mere line, and it has then the appearance of being the anterior 

 edge of the supposed tectum ; as it does not rise sharply on its 

 posterior edge, but slopes up gradually, it would seem to be a 

 natural result of the folding of the cuticle to form the projecting 

 end of the lamella. 



The last point I shall refer to is the exceptional mandible of 

 one single species. The ordinary mandible of the family is a 

 chelate mandible which, in all genera, except Pelops and the 

 instance I am about to mention, is broadest in the middle and is 

 terminated by a powerful chela (plate I. fig. 10). 



In Pelops there is marked difference in the shape of the mandible 

 (plate I. fig. 11): the base is broad, almost quadrangular, then 

 the organ suddenly narrows and is continued as a long thin rod, 

 terminated by a very fine chela ; this doubtless fits it for exploring 

 small holes. In every recorded instance the mandible is chelate, 

 and this is given as a leading characteristic of the family. 



One of Nicolet's species is Leiosoma mierocephala ; in external 

 appearance it is but slightly distinguished from other species of the 

 same genus. It is rare, and I had only two or three specimens ; it 

 is my habit to draw the mandible, but this necessitates breaking up 

 the creature, and as they usually vary but little I did not like 

 destroying a specimen ; from what I could see, however, I suspected 

 that there was something unusual in the trophi, and I sacrificed 

 one of the three ; to my astonishment I found a mandible (plate I. 

 figs. 12, 13, 14), totally different from that of all other Oribatidse 

 which I know of; having somewhat the form of the Pelops mandible, 

 but longer in the shaft (or thin portion), and curved instead of 

 straight, but not chelate at all, on the contrary, devoid of any 

 movable joint, and regularly serrated along the distal portion for a 

 considerable length so as to serve as a sawing instead of a seizing 

 or tearing organ. It seems to me that with this essential variation 

 it is not possible to leave it in the genus Leiosoma, and I propose 

 to create a genus to be called Serrarius for its reception. 



Finally, I wish to express my thanks to Mr. Charles Stewart, of 

 St. Thomas' Hospital, both for his encouragement to persevere in 

 what I feared might prove to be investigations too difficult for me, 

 and for his able opinions as to the results I had obtained ; and also 

 to Mr. E. Bostock, of Stone, and Mr. C. F. George, of Kirton 

 Lindsey, for supplies of living specimens which enabled me to 

 continue the research in London. 



