PROFESSOR PETERS ON THE GENUS PECTINATOR. 403 
longer, and the spine of the second (Echinomys) or third thoracic vertebra suddenly very 
much developed. In Pectinator the spines gradually increase and are reclined until the 
(ninth) diaphragmatic or central vertebra. The spines of the last three thoracic vertebree 
are, like those of the lumbar vertebra, directed forwards. All the thoracic vertebra have 
a median ventral ridge sloping backwards. 
<=... The shape of the short spines of the seven lumbar vertebre is also more like that of 
Habrocoma than that of Echinomys or Chinchilla; but the oblique processes (anapo- 
physis, Owen) differ from those of all these genera in their short and compressed form, 
more resembling those of Spermophilus (citillus). 
Four vertebra coalesce to form the os sacrum, only the first of which is joined to the 
iliac bones'; it differs by its narrowness, or by the shortness of the transverse processes, 
from that of all other allied genera, and indicates in this respect a certain affinity to 
the Arvicole. 
The caudal vertebree have also very short transverse processes. Only the anterior five 
are provided with short spinous processes. The first chevron bone is attached to the 
third, and the last to the thirteenth caudal vertebra. 
Both Pectinator and Ctenodactylus have, like Spalacopus, only six pairs of ribs joining 
the sternum, the other six being false ribs. From the second to the sixth the ribs are 
comparatively broader than in any other genus belonging to the Hystricide, Muride, 
or Dipoda. 
The sternum is much more flattened, and comparatively broader, than in any 
other Rodent. In its general form, principally of the manubrium, and in the develop- 
ment of the lateral and a pointed median episternal appendage, it comes nearest to that 
of Chinchilla. It is composed of five, or, from the coalescence of the fourth and fifth, 
only of four bones. 
The scapula has the bifurcate acromion characteristic of the Hystricide, and resembles 
in the length of the base of its spine much more that of the Chinchille than that of 
Echinomys, Habrocoma, and Spalacopus. But in general form, and in the position of 
the spine nearer and more parallel to its lower margin, it comes nearer to that of 
Sciurus and Spermophilus. 
The form of the humerus is in general the same as in Chinchilla, Echinomys, Spala- 
copus, and Habrocoma ; and it has the foramen supracondyloideum placed in the same 
manner; its anterior crest descends beyond the middle of the bone, as in Habrocoma. 
The bones of the forearm are also hardly different from those of Habrocoma. ‘The 
carpus is composed of four bones in the first, and of five in the second row, the os 
intermedium being placed between the lunatum, multangulum minus, capitatum, and 
the metacarpus of the index. The metacarpal and the single phalanx of the thumb are 
very short, and of the same length. The four other fingers are normally developed. 
each with three phalanges. 
* In Habrocoma, Chinchilla, and Echinomys the second sacral yertebra is also partly united with the iliac bones. 
