PliOCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 291 



valves make their appearance in the form of fine granulations. Soon 

 iifter tJiis, the animal can crawl as well as swim, and the mantle becomes 

 separated from the foot by an indentation. The eyes are placed on the 

 ventral side, and hardly \'isible from above. The upper anterior part of 

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

 The valves first appear in the form of seven narrow bands with irregular 

 margins; the tuft disappears. The head and mouth then develope 

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

 ha^•e 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-jjlate appears behind the seventh. 

 The 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 piece, 

 and not by the coalescence of jiarts 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. 



Ee>u(l Organs. — Middeudorf 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 

 the contraction of the tissues of my specimens by alcohol may have 

 obliterated it.* Schiff in C. irkcus did not detect any renal organ, and 

 unless in an abortive condition it seems probable 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 C. Icevis and G. cinercKS by Lovent in his original paper, 

 the latter of w^hich has been coi)ied by Gray;f a figure, intended to rep- 

 resent the radula of G. ^tellcri, by Middeudorf; § of G. jnceus by Schiff; |1 

 of "0. marmoratus'''' by Eberhard;^ and a figure of Ghitonellus sp. by 

 Gray. J Of all these onlj' the figure of Loven possesses any value, the 

 others being more or less erroneous, or conveying an erroneous impres- 

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

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

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

 proceeding justified bj" that single character; for the characteristics of 



*Dr. V. Ihering has sncceedcd in lintling an oritieo immediately below the anus, in 

 .some species. 



tOfv. K. v. Akad. Forli. June 9, 1847, t. 6. 



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



9>Beitr. Mai. Ross, i, pi. iii, f. U, 1847. 



II Zeitsclir. Wiss. Zool. ix, pi. ii, Beitr. zur Anat. v. Chiton piceus. 



UProgran^m Herz. Realscliule zn Cobnrg, 18G5, f. 77. (Since the above was written. 

 Prof. Sars has ligiu-ed the dentition of several species.) 



