104 MR. W. K. PARKER ON THE STRUCTURE AlHD 



8. The processes of tlie basal plate are all very slender, and there is a pair of small 

 extra-hyals. 



9. The squamosals have a very narrow supra-temporal part, and the vomers are of 

 great size, especially their dentigerous lobe. 



Second genus. Acris. 



44. (A) Acris Pickerincjii. — Adult female; 10 lines [^ inch) Lmg. Cambridge, 

 Mass., U.S. 



Although essentially a Ilijlodes, this minute Frog is w^orthy to be put into a sub- 

 genus ; I therefore retain for it the terra Acris. The skull of this species is very 

 valuable as .showing the effect of dwarfing in a remarkable arrest of the chondro- 

 craniura, such as is seen in the dwai-fs of other genera, in different and distant 

 regions. The small species of Rappia from AustraUa, e.g. R. hicolor, and similar 

 minute Oxydactt/le Frogs from the same territory, e.g. Camariolius, have the same 

 modification of their skull — almost all windows, with but narrow strips of wall ; and 

 this is also seen, but to a less extent, in the small Bombinator Toads of Australia, viz.: 

 Pseudoj^hryne. Hence it is evident that this economy of cartilage is due to the same 

 cause as the general arrest in size of these species. On the other hand, some of the 

 smallest kinds are most ossified, notably a species close akin to this, and to Rappia 

 hicolor, viz. : the small Rappia from Lagos ; and some of the small edentulous Anura 

 show the same thing, e.g., Ilylaplesia — a Platydactyle type : the smallest of the typical 

 Frogs, Rana pygmcea, has also a veiy bony skull. 



This skull (Plate 30, figs. 1-5) has its breadth slightly greater (one-twentyfifth) than 

 its length ; it is semi-oval in its facial outline, has a broad but rounded muzzle, and 

 its quadrate condyles (</.c.) end opposite the middle of the columella. 



The occipital condyles {oc.c.) are large, reniform, and postero-inferior ; they are 

 separated by a space neai'ly equal to the width of both, and this wide notch is 

 crescentic. 



There is a wide basal or median tract of cartilage, in which the remnant of the 

 notochord is seen to be enclosed in a feeble bony sheath — a cephalostyle — the rudiment 

 of a basioccipital {h.o.). Above the wide foramen magnum {f.m.) the roof-cartilage 

 comes well back, and is seen to be partly calcified, showing the rudiment of a supra- 

 occipital bone. This roof is complete up to the same transverse line as that from 

 which the optic nerves (II.) emerge ; from thence the roof is membranous nearly up to 

 the septum nasi {s.n.). This is the large ovoidal fontanelle (/«.) ; it is two-fifths the 

 length of the skull, and three- fourths the width of the interorbital region. The 

 ossification of the liiud skull is feeble, and by the two pairs of normal centres (pr.o., e.o.). 



The ex-occipitals (e.o.) flank the posterior canals {p.s.c.) above, and surround the 

 twin nerve-passages (IX., X.) below ; the prootics {pr.o.) climb over the ampullar of 

 the anterior and horizontal canals {a.s.c, h.s.c.) above, and half surround the foramina 



