_Comparisons._--From _Liomys irroratus alleni_, _acutus_ differs as follows: Color slightly darker brown on upper parts; size slightly less; posterior border of nasals V-shaped rather than truncate; frontomaxillary suture medially concave or straight instead of convex; interparietal subcircular (anterior border) rather than triangular; basisphenoid broader; tympanic bullae larger and more inflated. From _Liomys irroratus jaliscensis_ (topotypes), _acutus_ differs as follows: Color slightly darker brown on upper parts; size larger, without overlap, in external measurements and in basilar length, length of nasals and mastoid breadth; posterior border of nasals V-shaped rather than almost truncate; frontomaxillary suture medially concave or straight rather than convex; interparietal subcircular rather than quadrilateral; basisphenoid wider; tympanic bullae larger. From _Liomys irroratus pullus_, _acutus_ differs in longer body, shorter tail, slightly longer hind foot; all of upper parts, and especially upper side of tail, more brownish and less blackish; posterior border of nasals and frontomaxillary suture differing in same way as from _alleni_; interorbital region narrower in relation to length of skull; over-all length of skull greater; interparietal anteroposteriorly longer; tympanic bullae more inflated. [Illustration: FIGS. 4-6. Three views of the skull of the type specimen of _Liomys irroratus acutus_. x 1.] _Remarks._--This relatively large, dark-colored, spiny pocket mouse of east-central Michoacan differs from its geographic neighbors in V-shape of posterior border of nasals, semicircular shape of interparietal, medially concave maxillofrontal suture, wide basisphenoid and larger tympanic bullae. The latter character is not constant. Intergradation with _L. i. alleni_ is shown by specimens from Querendaro in which the shape of the interparietal is exactly intermediate between those of topotypes of the two subspecies and also in that the basisphenoid is wider than in _acutus_ but narrower than in _alleni_. Intergradation with _L. i. jaliscensis_ is shown, by specimen No. 120275 (U. S. N. M.) from Zamora, in shape of posterior end of nasals, direction of maxillofrontal suture, and shape of interparietal. In each of these features the specimen from Zamora is almost exactly intermediate between _acutus_ and _jaliscensis_. In large size of tympanic bullae and wider basisphenoid the specimen agrees with _acutus_, but otherwise is nearly as small as _jaliscensis_ to which it is here referred. Actually the specimen could, with almost equal propriety, be referred to either subspecies.