292 Dr. J. E. Gray on the peculiar Structure 
ceding table; but I shall content myself with indicating the 
following :— 
1. The analogous muscles in the hip-joint and shoulder- 
joint of the same side of the body are arranged in reverse 
order—thus confirming the opinion of Vicq d’Azyr that the 
left leg should be compared with the right arm, and vice 
versa. is 
_ 2. The marsupial muscles in no respect correspond with the 
obturators, but find their true analogues in a muscle whose 
direction lies between that of latissimus dorsi and the pectorals. 
This muscle (wanting in the Alligator and Crocodile) is found 
in the following animals :—the Armadillo, the Seal, the Otter, 
and other animals that dig or swim. 
3. The analogue of the obturators is found in the second 
pectoral of the birds, which acts as a levator humeri, and 
whose line of direction lies between the pectorals. This mus- 
cle may possibly be represented in the Cnscodils and Alligator 
by the pectoral muscle extended from the first sternal rib to 
the posterior edge of the coracoid. 
XXXVIII.—On the peculiar Structure and Function of the 
Spicules of Hyalonema. By Dr. J. E. Gray, F.R.S., 
V.P.Z.8., F.L.S., &e. | 
OnE of the chief reasons assigned for regarding the rope-like 
axis of Hyalonema as part of a Sponge, to which some speci- 
mens have been found attached, is that it consists of spi- 
cules which are composed of silica, and formed like the spi- 
cules of sponges. Prof. Max Schultze, Prof. Wyville Thom- 
son, and others compare them with the long filiform spicules 
of Huplectella. pe 
Zoologists and microscopists have overlooked the importance 
of a very marked peculiarity in the formation of the spicules 
of Hyalonema that is not to be observed in the yaa of any 
kind of Sponge that I have examined or seen figured. This 
is the more remarkable as the peculiarity to which I refer 
was mentioned when I first described the genus, and is figured 
by Max Schultze, Brandt, and Bocage, and, indeed, by all au- 
thors who have figured the genus; but these authors have not 
considered why the peculiarity existed and the bearing it has 
on the question of the structure of the animals to which the 
spjcules belong. | “4 ) 
The spicules of Sponges are formed of a number of con- 
centric layers round a central line, and they always have a 
perfect, more or less acute end, which is simple and formed of 
