292 PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 



the teeth in composition and general form (though not in number) are 

 somewhat similar to one another and differ from all other groups much 

 more than they differ among themselves. But other characters of 

 greater importance seem to turn the scale unequivocally in favor of a 

 somewhat wide, separation of these groups, and the term Docoglossa was 

 adopted by the writer some years since for the order containing the 

 Limpets alone. 



The teeth agree in number and in general character in all the genera 

 and subgenera of Chitons which I have been able to examine; compris- 

 ing about half of the groups recognized by the late Dr. Carpenter. No 

 large group of genera or subgenera remains of which some form has not 

 been studied. Hence we may reasonably infer, until the contrary is 

 proved, that all the genera agree in the most essential characters of the 

 dentition. Some doubt exists in my own mind as to the proper distri- 

 bution of the eight side-teeth into true laterals and uncinals, since the 

 fifth from the centre is constantly spatulate, yet separated from the 

 cuspid teeth by two boss-like or non-cuspidate teeth resembling uncini. 

 The formula therefore may be read either as 



1 1 1 



+ + 3 + 5x5 + 3' or 0+2x2 + 0' 



The most natural division is into six uncinal and two true laterals. 

 For convenience in description, I shall term the spatulate third uncinus 

 the major uncinus, the second lateral the major, and the first the minor 

 lateral. The " inner" side of a tooth is that toward the middle line of 

 the radula. In all Chitons examined, there is a simply cuspid rhachidian 

 tooth, and on each side a translucent minor lateral of varying form ; a 

 major lateral larger than any of the other teeth with a conspicuous black 

 cusp, which may have from one to four denticles ; two boss-like or thick- 

 ened uncinal plates of irregular shape; a twisted spatulate uncinal and 

 three scale-like or slightly thickened external uncini. With the excep- 

 tion of the spatulate uncinus (which is abortive in a very few species), 

 none of the uncini are much raised above the plane of the odontophore, 

 and none present any characters of importance. The characters of the 

 other teeth, though preserving a tolerable uniformity within the partic- 

 ular subgenera, so far as observed are rather variable within a certain 

 narrow range, and on the whole it would be premature to say that they 

 offer more than specific distinctions. 



The absence of any well-marked types by which the order might be 

 divided into families, or even subfamilies, is very remarkable, and in 

 this respect the variations of the dentition agree with the other charac- 

 ters of shell-plates, girdle, and internal structure. This has already been 

 remarked as regards the girdle and shell by Dr. Carpenter, who recog- 

 nized that even his chief divisions of the order into Regular and Irreg- 

 ular Chitons failed to posssess distinct family value. 



The only other dental formula which recalls in any degree that of Chi- 

 tonidw is that of Triopa lacer as figured by Gray in his Guide. 



