I 38 CEPHALOCHORDATA CHAP. IV 
Thus sixteen species have been described, of which the three 
under Branchiostoma placed after square brackets, seem to be 
merely varieties of B. lanceolatum, and B. nakagawae is probably 
identical with B. belcheri ; while it is a question whether Asym- 
metron caudatum is more than a variety of A. lucayanum, thus 
leaving eleven or twelve species that seem fairly well character- 
ised. The exact positions of the two marked (?), viz. B. elonga- 
tum and B. pelagicum, cannot be determined in the absence of 
fuller descriptions of these species. 
Kantords Cag 45tub! Londore 
Fic. 90.—Sketch-map showing geographical distribution of the Cephalochordata. 
+ indicates Branchiostoma; o indicates Asymmetron. 
The list above, and the map (Fig. 90), give some indication 
of the geographical distribution of the group, and show that, 
although the few species are widely distributed over the shallow 
waters of the globe, most of the records lie between 40° N. and 
40° S. latitudes. In fact the group is mainly a tropical one, 
and is most abundant in the Indo-Pacific region. The crosses 
indicate records of species of Branchiostoma, and the circles those 
of Asymmetron (including Heteropleuron) ; the latter are confined 
to the Indo-Pacific seas, with the exception of A. lucayanum 
from the Bahamas—one of the numerous cases of interesting 
similarity between the marine faunas of the East and West 
Indies. 
