2 CONTRIBUTIONS TO NORTH AMERICAN ICHTHYOLOGY IV. 
t Skull without membrane hones (a '' rudimental opercular hone” in CMrnara ) ; 
gills not free, the branchial openings slit-like, usually several in inrmher; 
exoskeleton placoitl, sometimes obsolete; eggs few and large. 
Er.AS.MOBRAXCini, o. 
it Skull with membrane hones ; gills free ; branchial openings a single slit on 
each side, sometimes conlluent; exoskeletou various, not placoid; eggs 
comparatively small and numerous Pisces, 4. 
Class I.-LEPTOCAllDII. 
{The Lmicelets.) 
Skeleton membraiio-cartilagiuons ; skull mideveloited, with the noto- 
chord persistent and extending to the anterior end of the head. Brain 
not differentiated. No heart; the function of the heart being jierforined 
by pulsating sinuses. Blood colorless. Bespiratory cavity continent with 
the cavity of the abdomen; branchial clefts in great number, the water 
being expelled through an abdominal pore in front of the vent. Jaws 
none ; the month a longitudinal fissure, with cirri on each side. 
Small marine animals highly interesting to the zoologist as exhibiting 
the lowest degree of develoinnent of the vertebrate type. The class 
includes but the single order Cirrostomi. thin; za/xJia, heart.) 
(Subclass Leiitocardii Giinther, viii, 513-514.) 
Order A.-CIRROSTOMI. 
{The Cirrostomes.) 
This order is equivalent to the family Branchiostomatidee. (Latin, 
cirrus^ a lock of hair; fTro/^a, month: the mouth being surrounded by a 
fringe of cirri.) {Cirrostomi Giinther, viii, 513-514.) 
Family I.— BRANCHIOSTOMATIDyE. 
{The LanceJets.) 
Body elongate, compressed, naked, colorless, with no fins, except a 
rayless fold extending along the back, around the tail, past the vent, to 
the abdominal jiore. Mouth inferior, appearing as a longitudinal fis- 
sure, siun'ornuled by conspicuous, rather stiff cirri. Eye rudimeutaiy. 
Liver reduced to a blind sac of the simple intestine. 
Genera two, Branchiostoma and Epigonoptenis, differing chiefly in 
