43 DESCRIPTIONS OF PREPARATIONS. 



suggestion as to placing Hyrax between Rhinoceros and Tapir is less open to 

 objection. It may be remarked that some of the peculiarities of Hyrax, such 

 as the bisacculate character of its stomach, the presence of a sterno-maxillary 

 muscle, the dilatations of its Eustachian tubes, the flatness of the roof of its skull, 

 the perfect orbital ring, the absence of a clavicle, and of an acromion on the 

 scapula, and the presence of a diastema between the incisors and the canines, 

 are points which are at least as indicative of equine affinities as of connection 

 either with the Rhinoceros or with the Rodent stock. And if on the other hand 

 the presence of a languette on the dorsal, and < papillae foliatat on the lateral 

 surface of the tongue in Hyrax, are points curiously reproduced in the Rabbit 

 and the Rat, the important point of the absence of a second superior vena 

 cava distinguishes Hyrax from all rodents except the Caviidae, and the zonular 

 character of its placenta distinguishes it essentially from all known Rodents 

 whatever. 



Few points of real affinity connect the Rodentia with the Insectivora in 

 addition to those more superficial peculiarities in general appearance, size, and 

 habits, which have caused the two orders to be connected in common language. 

 And these points are mainly such as may be characterized as indications of 

 comparatively low organization in the scale of Mammalian life ; and they are 

 rarely constant in all the members of the two orders. Among them we may 

 specify the not infrequent vacuolation or fenestration of the bony roof of the 

 palate, the imperfect condition of the bony support of the tympanum, and the 

 retention of the primitive jugular foramen. Similarly in the soft parts of both 

 orders we find usually, if not always, cerebral hemispheres devoid of convolutions, 

 and two superior cavae, as we do in all known Sauropsida. But irrespective of the 

 differences in dentition, which may appear to lose some of their importance since 

 we have been acquainted with the existence of Apatemys from the Middle Eocene 

 of America, which had (see Marsh, /. c. p. 43) gliriform incisors combined with 

 insectivorous molars, the digestive organs in the two orders are strikingly different, 

 the intestinal tract being provided with a caecum in all Rodents except the 

 Myoxini, whilst it is absent in all Insectivora except Macroscelis, Rhynchocyon, 

 and the Tupaiidae. Both orders alike, it is true, have a discoid deciduate 

 placenta, but in the Rodents the omphalo-mesenteric vessels take a large share 

 in the nutrition of the foetus up to the end of pregnancy, the umbilical vesicle 

 lines the whole of the chorion which is not occupied by the disk of the placenta 

 proper; and this disk is never attached except to the mesometrial border of the 

 uterine cornua ; whilst in the Insectivora as in the Chiroptera the umbilical vesicle 

 is attached, with the exception of Sorex, only over a limited circular area of the 

 chorion opposite that occupied by the true placenta, the functional importance 

 of its vessels is less, and the site of the allantoid placenta may be on any part of 

 the uterine walls. 



There can scarcely be any doubt that the Insectivora must be considered to 

 be as ancient a form of mammalian life as the Rodentia ; indisputable remains 

 however of the order have not as yet been found lower than the Middle Eocene, 

 whilst remains of Rodents have been identified in the lowest Eocene, the Cory- 

 phodon beds of North America. This absence of Insectivora however must be 

 ascribed to the present imperfection of the Geological record ; and it should be 



