96 GUIDE TO THE FOSSIL REPTILES, AMPHIBIANS, FISHES. 
majority being invested with thin and flexible deeply-over¬ 
lapping scales, which are either smooth (“ cycloid,” Tig. 104, a) 
or pectinated (“ ctenoid,” Fig. 104, b) at the hinder margin. 
A ’• B 
Fig. 104.— Scales of Teleostean Fishes, (a) Cycloid; (b) Ctenoid. The 
right border is exposed in the fish, and the grooved left border is deeply 
overlapped by the next scales. 
Wall-case 
15 . 
Table-case 
25 . 
Wall-case 
15 . 
Table-case 
25 . 
Wall-case 
10 . 
Table-case 
26 . 
Table-case 
20 . 
Next to the Leptolepidse in the collection are arranged the 
Elopidse, which are the Cretaceous and Tertiary fishes perhaps 
most nearly related to the highest Jurassic families. Among 
these, in Table-case 25, the specimens of Osmeroides from the 
English Chalk are especially noteworthy, several having been 
beautifully worked out of the matrix by the late Dr. Mantell. 
Like many fossil fishes from the Chalk, they are almost 
uncompressed, the fine chalky mud having replaced the soft 
parts as rapidly as they decayed, thus preventing the collapse 
of the flanks and preserving almost the natural form of the 
living animal. Thrissopater , Phacolepis and Pachyrhizodus 
are allied Cretaceous genera, while Elops and Megalops are 
Tertiary and still survive. 
Among the Albulidte may be noticed well-preserved 
skeletons of Istieus from the Upper Cretaceous of Westphalia 
(Wall-case 15), which can scarcely be distinguished from the 
existing deep-sea fish, Bathythrissa. In fact, many of the 
Westphalian Cretaceous fishes are related to living deep-sea 
genera, the eel-shaped Halosauridse ( Echidnocephalus ) in 
Table-case 27 being especially remarkable. 
The Chirocentridse, which are proved by their sole survivor 
to be closely related to the “ ganoids,” are also well repre¬ 
sented among Cretaceous fossils. The extinct forms are 
provided with powerful teeth implanted in distinct sockets 
on the margin of the jaw. Portheus attains a large size, as 
shown by a fine slab of Portheus molossus from the Kansas 
Chalk in Wall-case 16. More fragmentary remains of this 
genus, Saurodon, and Ichthyodectes are also exhibited from the 
English Chalk and Gault. 
The Cretaceous Ctenothrissidse are herring-like fishes 
which mimic the Berycoids in the anterior situation of the 
