1896. ] MAMMALIAN DENTITION. 567 
ppm.2, and fig. 14, ppm.3). The upper dpm.2 is larger than dpm. 2, 
possibly at times it cuts the gum ; it is well seen in a specimen in 
the British Museum (fig. 18, dpm. 2), probably the one Thomas 
described ; no trace of dpm.2 is seen in that specimen, and it is 
probable that this tooth is either shed in utero or absorbed. 
oes are quite normal, the milk representative being large, and 
the lingual growths of the dental lamina, which give origin to the 
enamel-organs of their successors, being conspicuously swollen 
(fig. 14, ppm.5). This tooth is somewhat similar in the two 
dentitions, but distinctly larger in the adult. 
Dorm gr — These are the largest and most complex of the pre- 
molars, and both exhibit conspicuous lingual specializations of the 
dental lamina, the enamel-germs of their successors. These germs 
are developed in front of the deciduous teeth, and although the 
lingually-placed dental lamina is continued back by the side of 
dpm. 4, it is uo longer swollen to form an enamel-germ. 
The Molars. 
In the stage examined two molars were present, above and 
below, but save in the case of ™. 1 no labial or lingual developments 
of the dental Jamina were to be seen. M.1, however, exhibited 
both a lingual and a slight labial growth, similar to those seen in 
Erinaceus. 
The Cusps. 
The molar teeth of Gymnura resemble those of Erinaceus in 
pattern ; like that genus they exhibit five cusps, which are stron gly 
developed, and in the upper jaw a well-marked cingulum, with a 
small anterior and posterior cusp, is present in addition; in the 
Jower jaw the paraconid is less developed than in Erinaceus. 
My feetal specimen was rather old for an exact determination of 
the cusp ontogeny, most of the cusps being well-formed. In m.1 
all five cusps were present, and had attained nearly their full 
development; the following is their order in size: proto-, meta-, 
para-, and hypocones, the smallest being the metaconule. M.2 
was less developed, and here the para- and metacones were the 
most strongly developed, while the protocone was present in the 
form of a large antero-external shelf, but hardly as yet developed 
into a distinct cusp, though the hypocone and metaconule had 
done so. 
Probable order of cusp-development :— 
HH 
. Paracone. 
. Metacone. 
. Protocone as a shelf. 
. Hypocone. 
. Metaconule. 
. Protocone as a cusp. 
> Or He CO bO 
