Text book of zoology / by J.E.V. Boas ; translated by J.W. Kirkaldy and E.C. Pollard.

Date:
1896
Catalogue details

Licence: Public Domain Mark

Credit: Text book of zoology / by J.E.V. Boas ; translated by J.W. Kirkaldy and E.C. Pollard. Source: Wellcome Collection.

Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.

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    diversity of form; the crown is short and tuberculate; or furnished witli low transverse ridges (Mouse, Rat) ; or each has two fangs, but the crown is longer and is folded both from above downward and also laterally; or again, the roots are quite short as compared with the long ])licate crowns; lastly, they often grow from persistent pulps, and iuv. provided on (iiich side witli dox^.p, pcn-pendicular folds Dental Formulae. which extend for some distance into the tooth, and are partially or entirely filled with cement. On the grinding surface, therefore, there are transverse or oblique stripes of enamel with cement and dentine between. Occasion- ally the molars with persistent pulps {cf., the molars of Elephants) are even divided into a series of perpendicular transverse plates with cement between them. This variety in the form of the teeth is correlated with a diversity of habit. The molars with short crowns are relatively little used, the others more or very much. The number of teeth is greatest in the Hares, p f-, m |-; in others it is more or less reduced from the anterior end of the series; even, as may be seen from the accompanying list, to the exclusion of all the premolars ;* only from quite a few forms {e.g., the Australian rat, Hydromys, belonging to the Muridae), one of the molars, namely, the last m^, is also absent. Whilst the articular facets for the lower jaw in most Mammalia are in the form of transverse surfaces or pits, in most Rodents there Hare . • V 3 T' m 3 3 Pika . . • }} 2 1 }> 3 3 Squirrel . • }> 0 t )} 3 3 Beaver . • }) 1 1 }} 3. 3 Sminthus • }} 1 0 }} 3 3 Mouse . • }) 0 0 }} 3 3 Australian Rat ' >} 0 0 }) 2 ■5 * As the correspondinfx milk teeth are also generally absent, and as (with the exception of the Hares) the incisors have no predecessors, there is absolutely no replacement in forms destiti^te of premolars,