ON THE STUUCTUKE AND CLASSIFICATION 
2nc 
with a prominent ringuhm and sometimes distinct basal cusps. Median innsors 
elongate, diminishing laterally. Condyle lofty. Goronoid slender. Angle distinct, 
ftosteriorly placed, not inflected. 
'J’lje fiimilv embraces Amhlotherium and Achyrodon, which are among the smallest 
genera of the Purbeck group. The mandibular symphysis is shallow and the mylo- 
iiyoid groove extends forward to it. As the inner surfaces only of the jaws are 
known there is considerable doubt as to the full structure of the teeth and of the true 
relationships of these genera. It is probable that some of the specimens which have 
been referred to Amhlotherium are, in fact, portions of Stylacodon. A suspicion as to 
the identity of these genera is raised by the mere fact that all specimens of Amhlo- 
therium present inner surfaces, while all the Stylacodon specimens present outer 
surfaces. Tlie proportions of the mandible are very similar but the incisor teeth are 
widely different. The determination of the systematic position of these genera 
tlepends ui)on the presence or absence of external cusps. The molars have a super- 
ficial resemblance to those of Amphitherium, but the distinction is very clear when 
the inner faces are compared, the Amhlothermm molars lacking the internal cingulum 
and conspicuous cingulum cusp. The family may be defined as above. 
AMBLOTHERIUM, Owen,i 1871. 
Dentition. — i c pm j, m The incisors are widely separate and semi-pro- 
cuml>ent. The canine has a single fang. The central molar cusps are rounded and 
rc'trovertcil at the tip. The premolars are slightly recurved and rise to the level of 
the middle molars. 
ACHYRODON, Owen,= 1871. 
Dentition i ., c ,, pm m g. The third and fourth premolars rise much above 
the molar level. The central molar cusps are sharply pointed and turned forwards. 
I he anterior cusps are also acute and the posterior cusps are more elevated than in 
the allied genus. 
STYLACODONTIDiE, Marsh,* 1879 . 
Upjwr molars with a single styloid internal cusp connected by divergent transverse 
ridges with a pair of external cusps, which are followed by a lower posteiior cusp or 
hed. Juicer molars reversing this pattern. Molars with two or three fangs set trans- 
oereely, xoithout internal cingulum. Incisors diminishing laterally, spatulate in typical 
’ “ Mesozoic Maniinalia,” p. 37. 
■lour. lif r”"' M“»«<>embrace»,fo*«.™d 
O«l.,opod. (inda, '■ '•I for . gep... of 
family name. •i;i,i 8 is further preferable to "n ^ be substituted as the 
taUhed l>oth frol i’.a I doubtfully distin- 
