66 SINCLAIR— THE SANTA CRUZ TYPOTHERIA. [April 24, 



Order TOXODONTIA Owen. 



Suborder TYPOTHERIA Zittel. 



A. Family Interatherid^. Median incisors rooted; third and fourth pre- 



molars not completely molariform, squamoso-mastoid region difated and 

 cancellous ; malar long and narrow, inclosed between temporal process of 

 maxillary and squamosal ; maxillary orbital ; carotid canal and foramen 

 lacerum posterius fused; tibia and fibula unfused distally; pes par- 

 axonic, digits II. and V. equally reduced and small, digits III. and IV. 

 large and of equal length ; astragalar trochlea bilaterally symmetrical ; 

 no naviculo-calcaneal facet; calcaneum with large fibular facet. 



1. Protypotheriuni. Dental formula f, \, |, f in close series. Lateral 



incisors unreduced; canine incisiform; upper molars with deep in- 

 ternal inflection and slight antero-external ridges ; Ms externally 

 bilobate; temporal bar of maxillary with slight descending process; 

 humerus with internal epicondylar foramen ; terminal phalanges 

 laterally compressed hoofs with slight clefts in manus. 



2. Interatherium. Dental formula -f, t> I' l» with diastemata between the 



lateral incisor, canine and first premolar, varying with the species. 

 P reduced, often wanting: upper molars with deep internal inflection 

 and prominent antero-external ridges ; M3 externally trilobate ; tem- 

 poral bar of maxillary with strong descending process; humerus 

 without internal qjicondylar foramen ; terminal phalanges laterally 

 compressed hoofs with or without clefts. 



B. Family Hegetotherid.e. Median incisors rootless; third and fourth pre- 



molars molariform; mastoid dilated inclosing a large hollow cavity; 

 malar large excluding maxillary from orbit ; carotid canal and foramen 

 lacerum posterius widely separated ; tibia and fibula firmly fused both 

 proximally and distally; pes approaching mesaxonic with digit III. the 

 longest, digit V. greatly reduced and digits II. and IV. shorter than 

 III. but robust; astragalar trochlea bilaterally asymmetrical; navicular 

 and calcaneum in articulation; small fibulo-calcaneal facet. 



1. Hegctothcritim. Dental formula f, \, |, f. Second and third upper 



and third lower incisor vestigial ; canine vestigial ; upper molars inter- 

 nally convex, without inflection except in M^ ; ectoloph smooth ; 

 terminal phalanges greatly flattened transversely with prominent 

 clefts. 



2. Pachyrukhos. Dental formula \, ^, f, f . All the upper molars inter- 



nally convex ; ectoloph smooth ; terminal phalanges hoof-like without 

 clefts in Santa Cruz species. 



The Santa Cruz typotheres are animals of somewhat rodent-like 

 appearance, varying in size from a cotton-tail rabbit to a cavy. A 

 review of the more important skeletal characters of the group may 

 be of value, even though it involve some repetition. 



