PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 283 



Limpets to be placed on a definite footing. That any classification 

 founded on single characters, or a small proportion of characters, may 

 result unfavorably to this view, I am not prepared to deny ; but what- 

 ever advantages such a method may present, it is not one which appears 

 worthy of the name of philosophical treatment, or likely to endure as 

 our knowledge becomes more definite and extended. 



It is not yet certain how far tlie indications of the dentition may be 

 relied on in this group, as will presently be shown. Whether, as in the 

 implacental mammalia, the teeth will prove an insecure basis for gener- 

 alization beyond genera, or whether a classification b;ised upon them 

 will present a more favorable aspect when a larger number of species 

 have been examined, it is yet impossible to say. What is known pre- 

 sents some anomalies to which the key is at present wanting. So far 

 as investigation has proceeded, greater weight seems due to the charac- 

 ter of the shelly plates than to any other single feature, and the tenta- 

 tive classification of Dr. Carpenter is in this way justified. Any division 

 of the group into families seems i)remature without more light. The 

 genera and subgenera are, in most cases, reasonably sm'e on their founda- 

 tions; but on an examination of the dentition, such as is contemplated by 

 ])r. Troschel, and is urgently needed, much will depend. But until this 

 has been made very full and thorough, it is to be hoped that generic 

 distinctions based on the teeth alone may be suspended, or at least left 

 without names. 



A sketch of the outlines of Dr. Carpenter's classification will not be 

 out of i)lace here, but is best preceded by an explanation of certain 

 terms used in description. 



In all Chitons with exposed valves, the seven posterior valves are 

 divided more or less jilainly by lines radiating from the apex to the 

 opposite anterior edge. The sculpture of the posterior triangular areas 

 {arece laterales) thus cut off" is almost uniformly like that of the whole 

 anterior valve and the part behind the apex {mucro) of the posterior 

 valve. The central or anterior triangles {arece centrales) are sculptured 

 alike, but generally in a different pattern from the sides. The arew la- 

 terales are usually raised a little above the rest. It is very rare that the 

 bounding diagonal lines cannot be traced, and they usually correspond 

 to the slit in the side-lamincc of insertion, which project into the zone or 

 girdle, and are free from the peculiar porous superficial layer character- 

 istic of the exposed test in the whole group of Chitons. This sui^er- 

 ficial layer usually projects over the anterior and posterior laminae of 

 insertion or teeth {denies) in the first and last valves, forming what Dr. 

 Carpenter terms the ' eaves' {suhgrundw). These may exhibit the spongy 

 character of the layer of which they are formed, or may be varnished 

 over at their edges with a thin layer of true shelly matter, as in the 

 Ischnoid group. In the t;sTical Chitons they are short, lea\iug the 

 teeth projecting; in the Mopaloids they are hardly developed, and in 

 some groups they quite overshadow the teeth. 



