1889.] NATURAL SCIENCES OF PHILADELPHIA. 27 



With this I close the present account of the cranium in this em- 

 bryo of one of our not abundant species of American rodents. Upon 

 some future occasion I trust to compare it quite extensively with the 

 development and morphology of the skull in other types of mammals 

 of this country, especially the rodentia. 



Explanation of Plates I and II. 



Note. — The figures of these two plates were drawn and colored 

 by the author directly from his dissections, and in them the mem- 

 brane bones have been simply shaded ; the cartilage bones colored 

 orange ; and the endocranium and other cartilaginous parts, purple. 



In all the figures the structures are increased six times the size of 

 life, and the following letters used as abbreviations : 



ag. p. Angular process. 



al. e. Aliethmoid. 



al. n. Alinasal. 



al. s. Alisphenoid. 



al. sp. Aliseptal. 



a. ty. Annulus tympanicus. 

 au. Auditory capsule. 



b. h. br. Basihyobranchial. 

 b. o. Basiocclpital. 



b. s. Basisphenoid. 



cr. g. Crista galli. 



cr. p. Coronoid process. 



a: pi. Cribriform plate. 



cl. Dentary. 



e. hy. Epihyal. 



e. n. External nostrils. 

 /. Frontal. 



f. m. Foramen magnum. 

 fo. Fontanelle. 



r/l.f. Glenoid cavity. 



i. Incus. 



i. />. Interparietal. 



i. tb. Inferior turbinal. 



j. Jugal. 



I. Lacrymal. 



I. c. Lacrymal canal. 



mk. Meckel's cartilage. 



