1887.] ANATOMY OF HYDROMYS. 55 



the under and inner part of the zygoma witliin the orbit, and partly 

 from the superior maxilla anterior to the orbit ; this last portion 

 joining the rest by passing between the superior maxilla and the 

 zygomatic process just mentioned. Tiiis 3r J part joins the anterior 

 part of the 2nd. The remaining facial muscles call for no special 

 notice. 



Sterno-mastoid, which is larger than cleido-mastoid, is inserted 

 by a small round tendon into tlie mastoid process. Cleido-mastoid 

 takes origin from tlie inner jiart of the clavicle, and is attached under 

 the last to the mastoid process. 



Steruo-hyoid : both muscles are united into a single sheet without 

 trace of median differentiation. The middle part is inserted much 

 higher up than the two lateral, viz. in tl>e angle bet.veen the two 

 anterior bellies of the digastrics, the lateral fibres being attached 

 below the tendinous part of the digastric and near the omo-hyoid. 

 The omo-hyoid itself has no central tendon. Digastric has no true 

 tendon, the central position connected with the hyoid beino- con- 

 stricted and covered on its superficial surface with a few tendinous 

 fibres. The two anterior bellies are connected with one another. 



It may perhaps liere best be noted that the two halves of the inferior 

 maxilla are extremely movable upon one another, a quantity of 

 fibrous tissue intervening at the symphysis so as to form a kind of 

 fulcrum by means of which a scissors-like action of the extremely- 

 long inferior incisors is obtainable (vide fig. .5). The teeth are 

 divaricated from one another partly by the action of the digastrics 

 and partly by a transverse iiitermandiiiular nuiscle (fig. .5, a), which 

 Hes above the insertion of the digastrics and quite separate from 

 them. It is placed at the upper part of the angle between the two 

 halves of the maxilla, and is attached to the inferior surface of either. 

 Approximation is produced, at least in part, by the masseter and 

 especially by the part described above as 1. From this it appears 

 that au interval could be produced between the two lower incisors 

 during the opening of the mouth which would disappear with its 

 closure. Murie and Bartlett \ in a paper on the " Movement of the 

 Symphysis of the Lower Jaw in tlie Kangaroo," give an excellent 

 account of the mechanism of this movement ni the Macropodidse, and 

 quote from Good's 'Book of Nature ' a statement to the effect'that 

 a similar movement takes place in Mus mariiimus, the African rat. 

 In Hydrcmys there is no such development of the transverse fibres 

 of the orbicularis oris as the above authors describe in Halmaturus 

 bennettii ; whilst the intermandibular muscle above mentioned is 

 quite distinct from any of the other inframaxilLiry muscles, all of 

 which are present in addition. The amount of divergence possible 

 would be from g to j inch. 



Muscles of Shoulder-girdle and Upper Extremity. — The two 

 portions of the trapezius are quite distinct, the origin of the 

 lower being partly from the lumbar fascia. There is a large 

 occipital rhomboid, and rhomboidei major and minor form a sin»le 

 sheet without sej.aration. Orao-cervicalis arises from the transverse 



1 P. Z. S. 1806, p. 28. 



