124 



NOTES ON AN EXAMINATION OF FOUR SPECIES OF 

 CHITONS, WITH REFERENCE TO POSTERIOR ORIFICES. 



By William H. Dall. 



1. Stimpsoniella Emersonii (two specimens). 



The large and fine specimen from the Gulf of St. Lawrence pre- 

 sented a posterior and terminal anus of large size, but with the edges 

 not elevated into a papilla. The head of an ordinary pin could be in- 

 serted into it without violence. 



The orifices of the ovaries, bilaterally symmetrical, were situated 

 just behind, and, as it were, under the shadow of, the last branchia 

 on each side. There were two fenestras on each side of the anterior, 

 a little further towards the girdle, and a little larger than the poste- 

 rior. 



This species resembles in most particulars the Symmetrogephyrus 

 Pallasii of Middendorf, and it would seem as if his ungainly subgen- 

 eric or generic name should be adopted. The hairs are precisely 

 similar in both species, as are the branchia?. The insertion plates 

 also agree, according to Dr. Carpenter, who examined a series from 

 a specimen obtained by me in the Aleutian Islands. The principal 

 difl'ereuces, beside the larger size of Pallasii, are as follows : In the 

 latter the hairs are more closely set, the texture of the epidermis is 

 thicker and harder, the points of the valves are more nearly covered, 

 and the skin is smoothly rounded over the back, not showing any- 

 thing of the form of the valves, as is the case in Emersonii. I think 

 also the valves are smaller, in proportion to the size of the animal, in 

 Pallasii than in Emersonii. 



2. Tonkella marmorea Fabr. 



This species showed a clearly defined posterior and terminal vent. 

 The fenestra? of the ovaries were symmetrical on each side, but the 

 branchiae pass behind them and conceal them; They are very small, 

 and I could not detect more than one on each side, though fresh 

 specimens, not hardened and contracted by alcohol, might show more. 



3. Trachydermon albus Lin. 



The same remarks apply to this species. The vent was terminal, 

 and on a papilla. 



4. Trachydermon ruber Lin. Three specimens examined. 



These specimens were much hardened by alcohol. Removing the 

 plates from above and then the inner lining membrane, beneath the 

 large and well tilled ovaries the intestinal canal is seen, terminating 

 in the median Hue posteriorly. From the outside the anus was not 



