THE LIMITS OF THE CLASS OF FISHES. Tt 
II. MARSIPOBRANCHIATES. 
ONYMES, AS CLASSES. 
<Ichthyoderes gga ra), Geof’. St. Hilaire, Latreilie, 1825. 
ontes, Agass., 1857 Gling Amphioxus ?). 
<Myz 
ia Cipatonbenta, prire 
=Dermopteri, Cope, cae 1872, 
SYNONYMES, AS SUBCLASS. 
ANRETT Bon., 18£0 (incl. eg aa 
Miiller iso et auct, plur 
< “ Bon 
<Dermopteri, Gin, 1861, aaa 1866. 
-> 
Ill. LEPTOCARDIANS. 
SYNONYMES, AS CLASS. 
= Myelozoa, Is, Geoff. St. Hil., Bon., 1856. 
< Myzontes, Agass., 1857. 
= Leptocardia, Häckel, 18% 
= Leptocardii, Cope, 1868, ne 
SYNONYMES, AS SUBCLASS. 
= Leptocardii, Müller, 1844, et auct. plur. 
<Dermopteri, Gill, 1861, Owen% 1838. 
These three classes appear to be better entitled to such distinc- 
tion than any of the higher classes of vertebrates, both on account 
of the greater taxonomic value of their distinctive characters, and 
their isolation from the next allied forms; and in both these re- 
spects, they are, for example, far more worthy of recognition than 
the classes of birds and reptiles as distinguished from each other, 
or the Pisces on the one hand and the Batrachians on the other. 
The difficulty, indeed, in the case of the inferior classes consists 
not in the inability to distinguish them from each other, but in the 
task of tracing the homologies between them, and there is no 
evident reason why this state of facts should not be to some 
extent reflected in the classification. The chief objection thereto 
of which the author is aware is a current and vague idea that what 
are called lower forms are more elastic and exhibit a wider range 
of variation than superior ones, and by assuming that all low forms 
of any branch, however much they may differ in structure, are 
constituents of a natural group to be compared with several more 
restricted higher ones ; such views may be enforced, but are liable 
to be considered, and only worthy of being treated, as petitiones 
principii. 
Certainly, the differences between Branchiostoma Amphioxus) 
and its nearest of kin are as evident and striking as those between 
