544A PROFESSOR E. RAY LANKESTER, 
To obtain from these the Limulus gill we have but to 
suppose certain definite changes of dimension, the imbrication 
and character of the lamelle, and their external position 
remaining unaltered (Pl. XXIX, figs. 2 a and 3 a). 
To arrive at the book-lungs of the Scorpion, we have to 
imagine the ventral surface on each side in close proximity 
to the short appendages carrying the gill-books—to have 
become deeply cupped or depressed, so that two series of 
cup-like pits should be formed, a right and a left, a pair 
being placed in each segment, correspending to each pair of 
gill-books. Each cup must have become so large in area 
and so deep as to embrace within its limits the relatively 
small adjacent gill-book (XXIX, fig. 26). Further, when 
once the gill-book had been involved in this cup-like de- 
pression, the walls of the cup must have tended to grow 
together so as to forma pulmonary chamber with only a 
narrow slit-like opening to the exterior (Pl. XXIX, fig. 3 4), 
and pari passu with this closing in of the cupped area, and 
the protection of the respiratory lamellz, the Arachnid must 
have acquired the power of leaving the water and of brew ing 
the atmospheric oxygen admitted to the damp «amber 
formed by the cave-like areas of depression. ae, 
Whilst framing such a hypothetical accoun! of the way in 
which the transition from naked “gill-bo.k’ to Intsunken 
« Jung-book” could have taken place, one naturally 
asks—“Is it not somewhat gratuitous ‘9 ® sume that 
cupped arez should form conveniently i side of 
the gill-books of the aquatic ancestor, so as ©) Le ready to 
increase in size, and ultimately draw into themselves, as it 
were, the gill-books?” ‘Is there,” we are led on further to 
ask, “ any known instance in Arachnida of the formation of 
cupped aree on the chitinous surface of the body? If so, 
can we show in what mechanical relation they are formed ? 
And, lastly, can it -be demonstrated that such mechanical 
relation probably existed in connection with the gill-books 
of the assumed common ancestor of Limulus and Scorpio ? 
If all these questions can be affirmatively answered, then 
our hypothesis as to the transition of the aquatic Arachnid 
to the pulmonate condition acquires great plausibility. 
The answer to these questions appears to me to have more 
than ordinary interest, since the formation of cupped arez 
on the chitinous surface of the body and the mechanical 
relations connected with their formation have, as pointed out 
a few pages back, come to light as demanded by the hypo- 
thesis. They exist in Limulus itself and in Thelyphonus. 
In Limulus there are two great muscles, a right and a left, 
