PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 291 



valves make their appearance in the form of fine .2:rannlations, Soon 

 after tliis, the animal can cra\yl as well as swim, and the mantle becomes 

 sei^arated from the foot by an indentation. The eyes are placed on the 

 ventral side, and hardly visible from above. The upper anterior part of 

 the animal is marked with acute tubercles. The month is not yet visible. 

 The valves first aj)pear in the form of seven narrow bands with irregular 

 margins; the tuft disajipears. The head and mouth then develoi)e 

 (Fig. 47 e). The eyes are on distinct lateral protuberances. No gills 

 have ai>peared. The mantle and front valve advance over the head 

 (Fig. -47 /) and eyes; the tuberculated area in front of the valves is 

 gradually diminished, and the tail-i)late apjiears behind the seventh. 

 Tlie valves are at first irregular, but increase from below, and deep 

 notches, persistent in the adult, are formed on the front edges, one on 

 each side. It will be seen that the valves are formed each in one i)iece, 

 and not by the coalescence of parts corresponding to the various areas 

 of the adult valve. There are eight valves in all Chitons, though mon- 

 sters with seven valves have been occasionally reported ; they lack the 

 horny jaw possessed by Limpets. 



Bemd Organs. — Middendorf indicated the existence of a renal organ 

 in the delicate glandular structure which in some species covers the up- 

 per posterior surface of the foot below the viscera. This does not seem 

 to be uniformly present; at least, I did not detect it in some cases, and I 

 failed to find any excretory opening. It is probable that this exists, but 

 tlie contraction of the tissues of my specimens by alcohol may liave 

 obliterated it.* Schitf in C. piceus did not detect any renal organ, anil 

 unless in an abortive condition it seems j)robable that it is not always 

 l)resent. 



Dentition. — The dentition of the Chitons has received hardly any at- 

 tention. The only figures which have been given, so far as known to 

 me, are those of ('. kvriti and C ei)tcreuH by Lovent in his original i)apei', 

 ■the latter of which has been copied by Gray;| a figure, intended to rep- 

 resent the radula of C. Stellcri, by Middendorf; § of 0. piceus by Schiff;i| 

 of "0. marmoratus''^ by Eberhard;<|| and a figure of Chitonellus sp. by 

 Gray. I Of all these only the figure of Loven possesses any value, the 

 others being more or less erroneous, or conveying an erroneous imi)reH- 

 sion. Even the number of teeth is not correctly represented by any 

 one but Loven. On the basis of the teeth, the Chitons were combined 

 with the Dentalia and Limpets in the order Docofflossa by Troschel, a 

 proceeding justified by that single character; for the characteristics of 



* Dr. V. Ihering has succeeded in fiudhig an oriUce immediately below the anus, in 

 some species. 



tOfv. K. V. Akad. Forh. June 9, 1847, t. 6. 



t Guide to Brit. Mus. 1857, pp. 182, 187. 



^ Bcitr. Mai. Ross, i, pi. iii, f. 11, 1847. ^ 



II Zeitschr. Wiss. Zool. ix, pi. ii, Beitr. zur Anat. v. Chiton picciis. 



^Programm Herz. Eealschule zu Coi)urg, 1865, f. 77. (Since the above was written, 

 Prof. Sars has figured the dentition of several species. ) 



