274 LECTURES ON MOLLUSCA. 



everything known up to the date of publication, illustrated by many 

 of the woodcuts in Davidson's work, will be found in "Woodward's 

 Manual of the Mollusca," pp. 209-240, and 465-467. Additional 

 genera are described by Prof. Hall in the annual Reports of the Regents 

 of the University of New York. Those who wish to examine magnificent - 

 series of the shells of the older rocks, exhibiting the internal structure, 

 are specially directed to the private collection of Prof. Hall, and to the , 

 Museum of the Geological Survey of Canada, arranged at Montreal 

 under the direction of Sir W. Logan. The following is a sketch of 

 the principal groups ; but as the distinctions of the genera depend 

 principally on the form of the lip-skeleton, which can be best under- 

 stood by figures, they will only here be indicated. 



Family Terebratulidje. (Lamp-Shells.) 



The Lamp-shells lie on their back, which is shielded by the smaller 

 valve; the front valve bends over, and is pierced at the beak by a hole 

 through which a peduncle anchors the animal to foreign objects. This 

 presents a fanciful resemblance to the plug of the Anomiads ; but, in- 

 stead of being a side-bunch, produced by the foot, it is a lump which 

 grows of itself behind the mouth ; as though a Chinese mandarin were 

 laid on his back and fastened by his hair-tail. So there is a resemblance 

 between the mouth-arms of the Palliobranchs and the mouth-feelers of 

 the four-gilled Cephalopods, Dr. Gray grouping these classes on each 

 side of the Pteropods ; but the likeness is almost as artificial as if we 

 should compare the Star-fish ivith the Cuttles, both groups having 

 locomotive organs round the mouth. 



Terebratula proper is thin and smooth, with a very short loop. This 

 only joins into a horseshoe ; in the striated shells of Terebratidina, it 

 unites into a ring. In Waldheimia, the shell is somewhat plaited, and 

 the loop is very long and reflected. Fudesia differs in being sharply 

 plaited. Meganteris is a long-looped Devonian form. In this group 

 the loop is attached near the end of the back valve. 



In Terebratella and its neighbors the loop is joined along the middle 

 of the valve, to a perpendicular plate. The cretaceous Trigonosemus 

 has a prominent, curved beak. Lijra (also cretaceous) has a long, 

 ribbed beak. Magas has the reflected parts of the loop disunited. In 

 Bouclwrdia the peduncle plate (called "deltidium," and separating 

 the hole from the hinge) is blended with the shell . Morrisia is moored 

 mouth-upwards, the hole being scooped out of both valves. 



Kraussia is a southern form, with the beak truncated. Megerlia is 

 also truncated, with the loop trebly attached. Ismenia has the valves 

 ornamented with corresponding ribs ; and Kingena has the surface 

 spiny. 



Family Thecidiad^i. 



Tliecidium has no hole, but is attached by the beak to sea-urchins, 

 corals, &c. Argiope resembles it in general aspect, but has a peduncle 

 through the truncated valve. The mouth-arms are folded into four 

 lobes; in Cistella, into two. Stringocephalus is a similar form from the 

 Devonian ; and Zellania resembles Tliecidium, from the secondary rocks. 



