THE SENSORY CANAL SYSTEM OF FISHES. 521 
dermo-ethmoid. They conduct the supra-orbital branch of the 
main sensory canal, and are bounded laterally by the dermo-ect- 
ethmoids. 
The Sub-orbital Series (Pl. 40, fig. 13, s. or.).—There 
are only two of the orbital scutes visible externally. The upper 
forms the posterior border of the orbit, and the lower both 
posterior and inferior borders. The sub-orbital branch of the 
sensory canal passes through both and then into the two 
internal ones lying anterior to the orbit. 
There is no pre-opercular scute in Acipenser, or any 
branch of the main sensory canal corresponding to the operculo- 
mandibular branch in Lepidosteus, &. What Parker (58) 
has termed pre-operculum is in all probability the jugal. 
There are no mandibular or maxillary branches of the 
sensory canal in Acipenser, the primitive pores in the imme- 
diate region probably functioning in their place. 
x. ACIPENSER STURIO. 
References to the sensory canal system of Acipenser are 
found in the writings of M‘Donald (52), van Wijhe (82), and 
others, but most of these are only in relation to the form or 
position of the scutes on the lateral portion of the body, con- 
ducting the lateral canal. 
Van Wijhe seems to have found some difficulty in tracing 
the canals, &c. His account is as follows :— 
“Was das System der Schleimcanale anbelangt, so sind die 
Rohre beim Stor ausserordentlich fein, nur mit Mihe kann 
man sie an einigen Stellen durchschimmern sehen. Um 
ihren Lauf wahrzunehmen wurden sie mit einer feinen Borste 
sondirt, und dann der Knochen, der sie umschloss, nach der 
Aussenseite aufgeschlitzt. 
“Der Hauptstamm liegt im Supraclaviculare, lateralen 
Supratemporale (Occip. externum, Gegenb.;  epioticum, 
Huxley) Squamosum, Frontale, asale Nund in der Scheide- 
wand beider Nasenlocher derselben Seite. 
“ Die supratemporale Quercommissur liegt hier, eben so wie 
bei den andern Fischen in den Supratemporalia, deren sich 
