BRACHIOPODA. 167 
as applied to those animals, because they are in the first place inapplicable to the 
character of the organism, and in the next they have never been universally employed in 
the same sense. 
The vent of the animal of the Brachiopoda is said to be situated nearer to the umbo 
than is the mouth, and on that account the distinctions called “ anterior’? and 
“posterior ’ have been applied. The outer portion of the shell covers the arms and the 
greater part of the mantle, while the viscera and vital organs are in the umbonal part of 
the shell; and I think, therefore, that such terms as visceral region for the inflated 
portion and brachial region for that which is occupied by the spiral “arm feet’ would 
better define the different positions of the animal, and could never be reversed or 
misapplied. ‘There is also great confusion, or, at least, want of unanimity, in the use of 
the distinctive terms “ventral”? and “dorsal” valves, the perforated one having 
sometimes been called ventral and at others dorsal, and wce versd. Professor M‘Coy 
has proposed that the name “entering valve”’ and “receiving valve”? should be given 
to the two pieces, and Mr. Davidson further suggests that the perforated valve should 
be called “dental valve,” and the imperforated one the “socket valve.’ However, as 
this author still distinguishes the two valves as “ventral” and “ dorsal”’ I have followed 
his example. 
In the Biwalvia the distinctions are founded upon the hinge furniture, either on the 
dental character, or upon the position of the connector, but in the Brachiopoda the 
distinctions are made dependent upon the apophysary system; viz. on the calcareous 
internal appendage for the support of their spiral “arm feet.”” The muscular system of 
the Brachiopoda also differs from that of the Bivalvia, nasmuch as the latter open their 
shells, as well as close them, by means of muscles, of which they have as many as eight, 
exhibiting a somewhat complex mechanical operation for the opening and closing of the 
valves ; and this arrangement appears to me to be far less simple than that of the elastic 
connector of the Bivalvia, and to effect the object in a far less easy way. Whether this 
be a proof of inferior or superior organization I am not prepared to suggest, but it does 
appear from it that the rule that nature adopts the simplest method for attaining a result 
is not one of universal application. 
The valves of the Brachiopoda are, with some exceptions, more or less of an ovate 
form, the longer axis being generally from the umbo to the opposite margin, and the 
length of the shell is measured in that direction ; the breadth, therefore, being at right 
angles to this line, and the depth having relation only to the tumidity of the valves. 
This is not in accordance with the measurement of the Bivalves as adopted by myself, 
but it is the mode that appears to be generally accepted, and I have, therefore, measured 
my shells by that standard. 
