XIII 



PHYLUM CHORDATA 533 



continuous ; the zygoma is incomplete, and the tympanic does not 

 usually form a bulla. 



The pectoral arch also varies a good deal in the different families 

 of the Insectivora. In the true Moles and their allies there is a 

 remarkable bone of cuboid shape articulating ventrally with the 

 presternum and dorsally with the humerus, and only connected 

 by a ligamentous band with the scapula. Its mode of formation 

 from a mass of cartilage to the anterior face of which the clavicle, 

 formed as usual in membrane, becomes applied proves that 

 this bone represents a pro-coracoid as well as a clavicle. In other 

 Insectivora this bone is not developed, and the clavicle is a distinct, 

 long and slender bone, but vestiges of the inner or ventral ends of 

 the coracoid and pro-coracoid may be recognisable. Sometimes 



Fio. 1185. Skull of Tenrec (Centetes ecaudatus). fr. frontal ; max. maxilla; pa. parietal; 

 p.max. premaxilla ; sq. squamosal. (After Dobson.) 



the " mesoscapular segment " (p. 500) is represented by a distinct 

 bone intervening between the outer end of the clavicle proper and 

 the acromion process. 



The humerus usually has a supracondylar foramen. In the 

 Moles this is absent, and their humerus is remarkable in other 

 respects, being short, greatly expanded at the extremities, with a 

 prominent deltoid ridge, and with two synovial articular surfaces 

 at the proximal end, one for the glenoid cavity of the scapula, 

 the other for the coraco-clavicle. The radius and ulna are com- 

 pletely developed in all and are usually distinct, but sometimes 

 fused distally. In the carpus the scaphoid and lunar sometimes 

 coalesce, sometimes remain distinct ; an os centrale is usually 

 present. In the Moles the manus is extremely broad, the breadth 

 being increased by the presence of a large, curved, radial sesamoid. 



In the pelvis the symphysis pubis is in some cases elongated, 

 in others short, and sometimes absent, the pubes remaining 

 separated by a wide median ventral cleft. A third trochanter is 

 sometimes represented by a ridge. The fibula usually, though 

 not always, fuses distally with the tibia. 



