16 MR MACDONALD ON THE ANATOMY 



at the inner side of the principal fang of the first or internal pleural series (Plate 

 II. fig. 7, i) ; whereas, if a smaller tooth is at all present in Atlanta, it is at a 

 corresponding distance to the outer side of the larger one (Plate II. fig. 8, i"). 



Firola (Lesueur). 



T have only met with one species o^ Firola (Plate I. fig. 7), which appears to 

 he equally plentiful in the tropical parts of the two great oceans, Pacific and At- 

 lantic. It is, I believe, F. Lesueurii, and about two inches long, or nearly the 

 same size as Carinaria Gaudichaudi (Plate II. fig. 4). The body is elongated and 

 cylindrical, with a full and rounded cephalic region, a muzzle reminding one of 

 an elephant's trunk, and a laterally compressed rudder-like tail. The filamentous 

 appendage in my first specimens was either absent or accidentally broken off", and 

 the latter accident happened to the head of several examples of this species 

 taken in the West Indies. No tentacula, or even frontal processes, were at all 

 visible ; but the eyes were well developed, and appeared to make a nearer ap- 

 proach to the eyes of Carinaria and Cardiapoda than do those of Firoloides. 



The acoustic capsules may be readily recognised by the brilliancy of the con- 

 tained otoliths, a little internal and posterior to the eyes; the auditory nerves being 

 about equal to the optic in length, but very much more slender (Plate I. fig. 8, g). 



A fan-like foot (Plate I. fig. 7, g) springs from about the middle of the under 

 surface of the \)0(\y, the rudimentary mesopodium (Plate I. fig. 7, f) being situated 

 on the free margin of the organ, somewhat nearer its anterior than its posterior 

 extremity. 



The visceral nucleus (Plate I. fig. 7, k) occupies a deep notch on the dorsal surface 

 of the body, near its hinder end, and is enveloped in a glistening fibrous coat, tinted 

 with a madder-brown pigment. It is rather small, as compared with the size of 

 the whole animal, but, being surmounted with distinct branchiae, one more indi- 

 cation is afforded of the propriety of placing Firola between Firoloides and Car- 

 diapoda ; and this conclusion is strengthened by the position occupied by the 

 sucker-disc of the foot, and also by the characters of the generative organs, which 

 indicate something intermediate between the two genera alluded to. 



The comparison of the lingual dentition of Firoloides, Firola, and Cardiapoda, 

 will not only show how intimately they are related, but afford some assistance 

 in determining their relative position with regard to the other Heteropoda. 



The rachidian plates of Firola (Plate I. fig. 9, o ; Plate II. figs. 5, 6) are quadri- 

 lateral in figure, but about three times broader than they are long; the dental points, 

 as in Firoloides, form a broad comb, with a stout central fang ; but the teeth gradu- 

 ally diminish in size towards the sides, and are so strongly marked, as compared 

 with those of Firoloides, that it would be impossible to mistake the one for the other. 



The first series of pleural plates very rarely exhibit a slight rudiment of the 

 small internal tooth so characteristic of Cardiapoda, Carinaria, and Ojcygyrus^ 



