OF CENTRAL CANADA PART IV. 251 



more copamon types are pedunculated, others are sessile. In the for- 

 mer, to which the well known barnacles belong, the animal is attached 

 to ships' bottoms, floating timber, &c., by a flexible, coriaceous stem ; 

 whilst in the latter, typified by the balanus or " sea-acorn," the shell 

 is fixed directly by its base to rocks and other submarine bodies, 

 especially to those which lie between the tide marks. 

 Fig. 164 shews the general form of a living bala- 

 nus with its cirrhi protruded between the smaller 

 opercula-like valves of its shell. Fragments of 

 comparatively large shells, which must have aver- 

 aged an inch or more in diameter, belonging to one FIG. 164. 

 or two species of Balanus ( B. Undevallensis, B. Livin s Balanus. 

 Hameri ?) occur in the Post-Cainozoic " Saxicava Sand Formation " 

 of Beau port near Quebec ; but no cirripeds are found in our lower 

 rocks, nor have any undoubted examples been discovered in Palaeo- 

 zoic strata. 



2. Ostracoda : The Ostracods comprise a large number of gene- 

 rally minute aquatic forms, in which the entire body is enclosed in a 

 bivalve shell, whence the name of "bivalve entomostracans " by which 

 they are often known. Natatory antennae, and several pairs of small 

 feet (which do not serve as swimming organs), project in living forms 

 beyond the shell. The latter is smooth in some genera, and more or 

 less embossed or tuberculated in others. Most living forms are ma- 

 rine, but some (Cypris, &c,) are fresh- water types. The best known 

 Palaeozoic genera comprise Leperditia and Beyrichia. In Leperditia, 



the shell is comparatively thick, with straight 

 dorsal edge ; and it commonly averages from one- 

 fourth to three-fourths' of an inch in length. Fig 

 165 is an example from the Trenton formation. 



FIG. 165. 



Leperditia Canadcngis, I n Beyrichia, the shell is very similar in shape 

 Nat ' Size ma r o e n t0n For ~ but much smaller, rarely exceeding the 12th of 

 an inch in length, and its surface is tuberculated or embossed. 



3. Phyllopoda : This sub-division may be made to include the 

 Copepoda, Cladocera, and Phyllopoda, proper small* aquatic types, 



* The large, phyllopod resembling types, Hymenocaris, Dictyocaris, &c. of early palaeozoic 

 age, are now separated from the Phyllopods, proper, and placed in a distinct group, the Phyllo- 

 rarida of Packard. As shewn by Packard and Claus, they appear to form a connecting link 

 between the lower and higher crustaceans : the Entomostraca and JUalacostraca of many 

 classifications. They form the sub-division Leptostraca of Claus. 



