Retneivs — Prof. H. F. Osborn — Molars of Perissodactyla, 317 



The shell originates in a spiral consisting of from two to two and 

 one-half slightly overlapping whorls, and ranging in diameter from 

 0'8 to 1 mm. ; thence it extends in a straight line, tangent to the 

 spiral, or sometimes slightly reflexed. The straight portion of the 

 shell rapidly increases in diameter from 0-38 to O'iO mm. at the 

 spiral, to about 1-5 to 2 mm. at 2 cm. length. Some of the 

 specimens were entire, and showed that the body-chamber occupied 

 about one-half the length of the shell. The siphuncle is eccentric, 

 and was found to lie near the outer margin of the spiral. 



The species was determined from an examination of the suture- 

 line which was traced from the simple form of the very young shell, 

 thi'ough forms of gradually increasing complexity, up to the typical 

 suture of the adult of Baculites compressus, Say. 



That this spiral portion should have hitherto escaped observation 

 can be easily accounted for by its small size and fragile character. 

 In all probability it was broken off long before the shell had attained 

 adult size, and is therefore to be met with only in shells which were 

 preserved in their immature condition. 



The description is given in the Proceedings of the Academy, 1891, 

 part i. pp. 159-160, and is accompanied by figures of the young 

 Baculites, and of a series of their suture-lines showing the gradual 

 develo^Dment from a comparatively simple form to the typical suture 

 of the adult of Baculites compressus, Say. 



IS E ^V I E "VsT S. 



T. — Professor Osborn on the Molars of the Perissodactyla. 



MANY of our readers are probably acquainted more or less fully 

 with Professor H. P. Osborn's elaborate and interesting 

 researches into the nature of the primitive plan on which the 

 molar teeth of mammals are constructed, summaries of which have 

 appeared from time to time in several English scientific journals. 

 The result of these researches was to show that this primitive type 

 of tooth was of the so-called ' tritubercular ' form. It may not be 

 out of place to remind our readers that such a tritubercular tooth in 

 the upper jaw is composed of an inner tubercle, or protocone, and 

 of two outer tubercles respectively designated paracone and metacone. 

 By the addition of a second inner tubercle — the hypocone — and of 

 two intermediate ones, known as the protoconule and metaconule, 

 such a tritubercular molar becomes converted into the sextubercular 

 one occurring in many Ungulates. 



Professor Osborn has recently been engaged in investigating how 

 the more complex molars of the specialized Perissodactyles have 

 been evolved from this sextubercular type ; the results of this 

 investigation being published in a memoir issued under the auspices 

 of the Museum of Harvard College.^ Since these results are of con- 

 siderable morphological importance, and are especially interesting 

 to palaeontologists, a short resume of them may be acceptable to our 

 1 Bull. Mus. Comp. Zool. Harvard Coll. vol. zx. pp. 87, e6 seq. (1890). 



