MOLLUSCS. 
29 
■scales of the modern American Bony Pike (Lejndosteus), and 
the African Bichir {Polypterus), alongside those of certain extinct 
forms. A specimen of the scales of the Tarpon illustrates the 
largest development in point of size of the modern horny 
" cycloid " type. Spines of the Porcupine-fish show an extreme 
development of this kind of structure. Diagrams and spirit 
preparations illustrate the mode of attachment and succession 
of fish teeth. A large series of the teeth of Sharks and Pays 
displays the gradual passage from those of the ordinary 
cuspidate form to others arranged in a pavement-like manner 
and adapted solely for crushing. Both types occur in the 
Port Jackson Shark (fig. 12), while those of some Pays 
are solely of the pavement modification. Very remarkable is 
the dental structure in the Parrot-fish. The north end of this 
side of the case shows the various modifications assumed by the 
teeth of the modern Bony Fishes ; among which, as exemplified 
by the Wrasse, teeth are developed on the bones of the pharynx. 
Throughout this case specimens, or models, of the teeth of 
extinct fishes are placed side by side with those of their nearest 
living relatives. 
The wall-case on the right side of this bay shows the 
developmental history of various fishes, together with the 
form and structure of the gills, brain, heart, digestive system, 
and other organs. 
A small case affixed to the wall at the back of this bay 
illustrates the structure of the Lancelet, by the aid of spirit 
specimens, enlarged models, and coloured diagrams. One of the 
most remarkable features in the structure of this strange and 
primitive little creature is the atrial cavity, enclosing the large 
and complex pharynx. 
Leaving Bay VI., next the principal staircase on the east side Bay Vll. 
of the Central Hall, which, as mentioned above, is reserved for ^o^^^cs. 
recently acquired specimens of more than ordinary interest, 
the visitor passes to Bay VII. The wall-cases on both sides of 
this bay are devoted to Molluscs, or " Shell-fish." In the first 
division of the case, on the left wall, is mounted a series of 
specimens in spirit. 
Alongside is a large series of shells arranged on tablets to 
show : firstly, the leading structural types, such as univalve, 
