Davis—On the Fossil Fishes of the Chalk of Mount Lebanon. 593 
in Westphalen,” p. 37, pl. 1v., figs. 1-5 (‘‘ Paleeontographica,” vol. xi., 1863), com- 
bined the two species Istieus grandis and I. microcephalus of Agassiz under the 
new specific name I. macrocoelius, the acquisition of a large series of examples 
having demonstrated the identity of the two species. The remaining specics 
described by M. Agassiz are corroborated by additional specimens, and Dr. von 
der Marck institutes the new species Istieus mesospondylus from the Plattenkalke 
of Sendenhorst, in Westphalia. The species now described from the chalk of 
Sahel Alma will, consequently, be the fifth belonging to this genus. It is 
extremely interesting in affording evidence of the close relationship of the fauna 
of the chalk of Westphalia with that of Mount Lebanon—a matter which will be 
treated at greater length in another part of this memoir. 
Istieus lebanonensis, Davis. 
UPIS xox, tig./3.) 
This ichthyolite is sufficiently well preserved to show the form of the body, 
the character of the scales, and the arrangement of the fins; the vertebral column 
is well preserved, but the anatomy of the head and the spinous processes are 
defective. One of the most important characters consists in the extremely long 
dorsal fin. The fish is 5 inches in length from the snout to the base of the tail. 
The latter adds 1°5 inch to the length. The head occupies rather more than 
one-third of the length: it is 1:8 ich. The greatest height of the body is 
1-4 inch, immediately in front of the dorsal fin. The height gradually diminishes 
to the peduncle of the tail, where it is 0°45 of an inch. 
The head is large, but, unfortunately, not easily defined. The snout is some- 
what obtuse. The orbit high and well advanced. The gape was probably fairly 
wide, the mandible of the lower jaw being fully 1 inch in length. The operculum 
was large, rounded, and extended from the cranium with an oblique posterior 
curvature to the pectoral fin. Branchiostegal rays not exhibited. 
The vertebral column consists of a large number of closely-impacted vertebree, 
0125 of an inch in diameter across the circumference, and half the diameter in 
length. Seventy vertebre can be counted from the tail to the base of the anterior 
ray of the dorsal fin, and, allowing for the additional length to the occipital 
condyle, the total number of vertebree was probably eighty-five; of these, thirty- 
four were caudal. ‘The neural and hzmal spines are only exhibited near the tail, 
where they are the length of the vertebrae apart, and tend obliquely towards the 
caudal fin. ‘The ribs cannot be distinguished. 
The dorsal fin commences 0°5 of an inch behind the head, and extends 
2 inches along the dorsal surface. Its posterior rays are separated by a space 
of 1 inch from the caudal fin. It contains thirty-nine rays, supported by inter- 
