new Forms of American Rotifera. 21 



slightly concave, the dorsum arched, conspicuously or not, 

 tectiforra when the central, longitudinal, ridge-like elevation 

 is present, as it may or may not be ; two low, lateral, poste- 

 riorly converging ridges conspicuously or obscurely developed 

 or entirely absent, but when present the enclosed more or 

 less obovate dorsal space surrounded by tliem is flattened ; 

 frontal dorsal margin evenly but deeply concave ; ventral 

 border excised much as in Metopidia collaris ; frontal angles 

 acuminate, not produced into spinous processes; dorsal poste- 

 rior border very slightly concave ; posterior ventral margin 

 deeply excised, the lateral borders of the excision diverging 

 or parallel ; collar-like frontal band usually present, some- 

 times coarsely and abundantly punctate, sometimes entirely 

 smooth ; foot and toes essentially as in Metopidia collaris ; 

 internal organization not apparently differing from that of the 

 type. 



Length about y^o i'^ch ; length of lorica alone -g^Q- ; toes 



This differs from the type in the deeper concavity of the 

 front of the dorsum, in the reduction of the lateral spines to 

 mere acuminations, in the absence of the slight posterior 

 acumination terminating the posterior ventral excision, and in 

 the smaller size, although the size alone is of little importance. 



Internally there appear to be no prominent differences, 

 except that I have not positively observed the peculiar 

 arrangement of the lateral canals within the dorsum which 

 seems so characteristic of M. collaris; but these often are 

 obscure in that species when the ovarian germs are well 

 advanced or when the eo:,g is well developed, or sometimes 

 even when the alimentary canal is gorged with food or with 

 its remains. 



Monostyla hamata, sp. n. 

 (PI. Vll. figs. 6, 7, and 8.) 



Lorica broadly ovate in outline ; front differing in form on 

 both the dorsal and the ventral surfaces, as shown by figs, 6 

 and 7 better than any verbal description could do. The 

 figures were drawn with the camera lucida, and represent the 

 lorica frontally dilated as it is when the enclosed body is 

 retracted ; when the animal is extended the elastic front is 

 narrower than here shown, and the two acuminations on each 

 side of the dorsal excavation become less conspicuous ; foot 

 and toe together about one half as long as the lorica ; toe 

 not shouldered. 



When the body is extended it bears on its front dorsal 

 margin three sub-semicircular membranous lobes (fig. 8), the 



