AN UNDESCRIBED A CRANIATE. 237 



these, there is no doubt, I think, that such enumerations may be 

 relied upon as giving safe specific criteria though they should not 

 by any means be the only ones, if live specimens can be observed. 

 This in spite of the great difficulty in making accurate counts and 

 allowing for the probable, slight variations within the species. 

 Until the various forms can be monographed from a study of living 

 specimens such characters will be most useful. 1 



Admitting that the number and arrangement of the myotomes 

 furnish criteria of specific value in the group Acraniata, we must 

 concede that such peculiarities of the fins and reproductive organs 

 as we have found in the Bahama Amphioxus are of at least generic 

 value and justify the establishment of a new genus to be charac- 

 terized as follows : 



Asymmetron g. n. 2 



Gonads present only upon the right side. 



Ventral fin with no fin rays or successive fin-ray chambers. A 

 long caudal process. 



The only known species, Asymmetron lucayanum* may be charac- 

 terized as follows : 



Right metapleuron continuous with the median ventral fin, which 

 passes to the right of the anus. Pre-oral hood extensive, the cirri 

 united by the membrane throughout the greater part of their length 

 and smooth, without sensory papilla. Gonads on the right 29, ex- 

 tending from the 15th to the 43d myotomes inclusive. Myotome 

 formula 44 . 9 . 13 = 65. Length 13 mm. "Olfactory pit" ap- 

 parently absent ; nephridia and brown canals not demonstrated. 



Adults and young swimming at surface in the evening 'in June 

 and July at Bernini and Nassau, Bahamas. Also taken buried in 

 the calcareous sand. 



With this addition we may form the following summary of our 

 knowledge of the classification of the group Acraniata. 



1 For the aid of any who may wish to know of the existence of preserved speci- 

 mens, a list of those in the Smithsonian Museum is given in Appendix II. 



2 a-sv/u/ufrpos, 077 = wanting symmetry. 



3 From the Lucayas; the islands discovered by Columbus in 1492, now the 

 Bahamas. 



