Davis—On the Fossil Fishes of the Chalk of Mount Lebanon. 473 
of the existing Scyllium, the apex of the denticles being inclined at a sharp angle 
to the base of the teeth. 
The genus Thyellina, Munst., differs from Palzoscyllium in the following 
particulars, some of which have been pointed out by Dr. von der Marck. The 
body is not so high, but presents a more slim appearance; the head is smaller; 
the teeth are longer, straighter, and more sharply pointed; the vertebre of the 
spinal column are higher than broad; the pectoral fins are smaller in proportion 
to the size of the body; and the anal fin, which, in Palzoscyllium, is closely 
contiguous to the caudal, is placed in Thyellina, midway between the tail and 
the ventral fins, and, instead of being quite behind the posterior dorsal fin it is 
slightly in front of it; the last may, perhaps, be considered the most important 
difference. 
Thyellina is readily distinguished from Rhinognathus by the tenuity of its 
body and the small size of the head, as compared with the large head and 
long snout of the latter. The teeth of Notidanus gracilis are sufficiently cha- 
racteristic to separate it from that genus in addition to its altogether different 
external appearance. 
The species now to be described from the soft chalk of Sahel Alma appear 
to be more closely related to the genus Thyellina; the long, slender body, the 
form of the teeth, and the position of the fins in the one, and the finely-pointed 
teeth, and the advanced position of the anal fin in the other, are, without doubt, 
indicative of such relationship, though it is only after some hesitation that this 
determination has been reached. 
Thyellina elongata, Davis. 
(Pl. x1v., figs. 2, 3.) 
Left side of the body exposed, outline complete, with the exception of the 
anal fin, which has either been removed, or more probably is of small size, and 
folded beneath the body of the fish. The total length of the specimen is 9°7 
inches. The depth of the body is greatest behind the pectoral fins, being 0:5 of 
an inch, and it becomes smaller towards the tail, the pedicle of the latter being 
only 0°25 of aninch. Of the entire length, the head, exclusive of the branchial 
apparatus, occupies only one-twelfth; the first dorsal fin is 3:2 inches behind the 
head, midway between the extremity of the snout and that of the tail. The 
second dorsal fin is 1°75 inches behind the first. The pectoral and ventral fins 
are well defined; the latter is situated considerably in advance of the first dorsal. 
The body was enveloped by a thick skin, covered with dermal ossifications, 
arranged in definite order, presenting something of the appearance of an imbri- 
cated tesselated pavement. 
