. Bulletin. Science; Natural history; Natural history. ^MCA PC. Fig. 2. Geomyid arteries in the Circle of Willis; A. Pappogeomys; B. Geomys: C. Thomomys. AC A - anterior cerebral, BA = basilar. IC = internal carotid. MCA = middle cerebral. PC = pos- terior communicating. PCA = posterior cerebral. SCA = superior cerebral. present, is usually at the posterior margin of the bulla, and the opening for the internal carotid artery is usually just in front of this, between the bulla and the basioccipital bone. The more anterior positioning of the internal carotid artery, as found in the geomyids, is
. Bulletin. Science; Natural history; Natural history. ^MCA PC. Fig. 2. Geomyid arteries in the Circle of Willis; A. Pappogeomys; B. Geomys: C. Thomomys. AC A - anterior cerebral, BA = basilar. IC = internal carotid. MCA = middle cerebral. PC = pos- terior communicating. PCA = posterior cerebral. SCA = superior cerebral. present, is usually at the posterior margin of the bulla, and the opening for the internal carotid artery is usually just in front of this, between the bulla and the basioccipital bone. The more anterior positioning of the internal carotid artery, as found in the geomyids, is unique among rodents, and as far as we can tell, among mammals. Despite the odd position of the internal carotid in geomyids, there can be no question as to its identity. There are no other segments of the embryologic aortic arch pattern (from which the adult carotid pattern is derived) that could have been retained to account for this change in position. Although, the carotid canal is usually defined as an opening between the basioccipital and the perotic and tympanic bones just anterior to the jugular foramen or posterior lacerate foramen (see Hill, 1935; Wahlert, 1974), we also favor use of this name for the opening by which the internal carotid enters the skull in geomyids. Despite the anterior position of this opening, its homologies are with the carotid canal. The arterial pattern found in the Geomyidae is the same as that found in the Castoridae and similar (except for the origin of the internal maxillary artery) to the pattern in the heteromyids. The original finding by Guthrie (1963) that the internal carotid artery was missing in Geomys was in error, due most likely to poor dissection material. Elimination of this apparent variation among the geomyids brings their arterial pattern more closely in line with that found in their closest relatives and strengthens the finding that while much variation in the carotid artery pattern occurs between rodent families, these patt
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