. on the Principle of Cephalization. 27 
The Monomyarians combine in their structure both high =" 
low characters. In their open mantle, and certain other features, 
they rank high. In their fixed position, the attachment gener- 
would indicate the presence of both anterior and posterior ad- 
ductors, combined in consequence of the excessive shortness of 
their antero-posterior diameter. The Monoyarians present sin- 
gular features of xoialeery with the Prachicwiodk. Thus they are 
generally inequivalve. The viscera are compacted toward the 
dorsal region, and, when attached, they are generally by a pro- 
cess from the dorsal portion (e. g. Ano mia), the lowest feature of 
attachment. In all these instances; particularly with Anomia, 
the analogy is very striking; it is analogy only, and nothing 
more, for in their whole structure, and in the relative propor- 
tion of their diameters, they present just the opposite extreme. 
While we have in Brachiopoda the growth laterally, that is, 
spreading on the sides and depressed dorsally, and the valves, 
dorsal and cahkiad, in the Monomyarians we have the other 
extreme; the valves are right and left, and the display is hi 
the side, ‘the growth extending ventrally as it were. Sona 
are they that in certain forms, Placuna for example, it is Seok 
impossible to conceive the presence of soft parts between the 
valves. We compare the relative diameters between the Brach- 
iopods and Monomyarians, to show how unlike they are in this 
respect. 
ameter. BRACHIOPODS. _ Monomyarians, 
Antero-posterior. edium Small. 
Dorso-ventral. Small. Very large. 
Transverse Large. Very small. 
By reason of their excessive narrowness, the greater na 
of Monomyarians lie on the right or left valve, and as their 
sae as in oni fae paper on Eee ic ga 0 
Unionide, Journ. Acad. Nat. Sci., 2d Series, vol. iv, Be D- 
