48 



bulletin: museum of compaeative zoology. 



outline and occur one for each chamber. There are as a rule four fin-ray 

 chambers to a myotome. 



Anteriorly the dorsal fin passes over into a small rostral fin. Posteriorly it 

 is continuous with the much reduced caudal fin which invests the long uro- 

 stvloid process dorsally and ventrally as a very narrow blade. The caudal fin 

 is continuous with tlie rather broad ventral fin, in which there are neither fin- 

 ray chambers nor fin-rays. 



The myotome formula may be given as 43 -{-b + 14+ = 62+. Between 

 the anterior end and the atriopore the number of myotomes varies from 42 to 44, 

 and between the atriopore and the anus it is usually 5. For the region posterior 

 to the anus an exact number cannot be given, for the reason that the most 

 posterior myotomes are so small that it is impossible to count them or deter- 

 mine with certainty where the system terminates. In the enumerations for 

 this region given in Table 4, only the numliers that could be counted with 

 certainty are given, though in every case more myotomes were prol)ably present. 

 This is indicated by suffixing a plus sign to the numbers for this region and to 

 the totals. 



A well marked chorda passes through the animal almost from end to end, 

 and forms posteriorly a delicate, long, urostyloid process. 



The nerve tube has a distinct anterior eye-spot, and a series of numerous 

 smaller spots extending from the third myotome to about the region of the anus. 



The gonads, which vary in number from twenty-three to twenty-eight, form 

 a single series on the right side. In a specimen with twenty-six gonads they 

 extended from the eleventh to the thirty-seventh myotome. 



A summarized statement of the chief structural features of the five speci- 

 mens examined is given in Table 4. 



TABLE 4. 

 Structural Characteristics, etc. of A. macricaudatum. 



The presence of gonads only on the right side of the body and the well 

 developed urostyloid process place this species unquestionably in the genus 

 Asymmetron. It differs from all known species of this genus in the form of its 



