A. caudatum (Willey)—Myotomes 40 + 9 + 11: Louisiade Archipelago.
A. (Heteropleuron) bassanum (Günther)—Myotomes 45 + 16 + 14: Bass Straits, Australia.
" cingalense (Kirkaldy)—Myotomes 39 + 16 + 8: Ceylon.
" cultellum (Peters)—Myotomes 32 + 10 + 10: Torres Straits, Australia, Ceylon.
" maldivense (F. Cooper)—Myotomes 45 + 16 + 12: Maldive Archipelago, Zanzibar.
" hectori (Benham)—Myotomes 53 + 19 + 12: New Zealand.
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 Asymmetron caudatum is more than a variety of A. lucayanum, thus leaving eleven or twelve species that seem fairly well characterised. The exact positions of the two marked (?), viz. B. elongatum and B. pelagicum, cannot be determined in the absence of fuller descriptions of these species.
Fig. 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.