LECTURES ON MOLLUSCA. 119 



Inlet. The form of the ligamental area is an important guide in the 

 discrimination of species. 



Another very abundant group resemhles a flattened Cockle, with the 

 beaks nearly close and the hinge-line curved. Pectunculus has a lig- 

 ament like Barbatia, with very strongly marked muscular scars. The 

 inner margin of the valves is crennated, as in the Fan-shells, and the 

 free borders of the mantle have rudimentary eyelets to correspond. 

 The lips are simply a prolongation of the gills ; and the foot is large 

 and crescent-shaped, waved on the sole. They are probably more 

 active than the Arks. Half the species known are from the American 

 shores, where they range from shallow water to a hundred fathoms. 

 They first appear in the Neocomian age. The oldest shells of this 

 group, being found from the Bath Oolite, have the ligament concen- 

 trated in a pit between the beaks, like Lima, and are thence called 

 Limopsis. A few species are still living in the Old World, from Nor- 

 way to the Cape. As Macrodon and Lunarca are to the Arks, so is 

 the little crag fossil Nucinella to Pectunculus. On one side of the 

 hinge the teeth are broken up, while on the other the plain ridge 

 remains. A very similar shell has just been found living at Cape St. 

 Lucas, by Mr. Xantus. 



Family NUCULDXE. (Nut- Shells.) 



The shells of Nucula are like a small, angular Pectunculus, with a 

 pearly layer within. The cartilage is in an internal pit, and the 

 hinge is in two divergent rows of very sharply interlocking teeth. 

 They are generally covered with a smooth, horny skin, while that of 

 the Arks is shaggy, and of Pectunculus velvety. The foot is very 

 large, deeply grooved ; spreading out to crawl into a broad disk with 

 saw-like edges. The mantle flaps are freely open, without pipes ; and 

 the plume-like gills are small, and united behind. The lips are very 

 long, curiously ornamented, and capable of protrusion t outside of the 

 valves, forming a singular contrast to the Arks, with which they are 

 generally associated. The Nuculas are found in deep water and in all 

 seas ; they date from the earliest rocks, and are very numerous in 

 species. Nuculina, from the French Eocenes, resembles Nucinella, 

 but with an internal ligament ; while Stalagmium and Nucunella form 

 transitions to Limopsis. 



Family LEDID^E. (Beaked Nut-Shells.) 



This family, in most respects closely resembling the Nut-shells, and 

 like them having the mantle freely open, presents us with the strange 

 anomaly of a pair of regularly formed siphon pipes, reminding one of 

 Pandora and the Anatinids. The shell of Leda is like a beaked 

 Nucula, with a slight mantle-bend. The pipes are unequal and par- 

 tially united ; there being two flaps from the mantle which fold together 

 like a third tube. The species are found in deep water from all seas, 

 and abound in most ages from early times. Yoldia, which is almost 

 entirely a boreal form, has the pipes united,, with a deep mantle-bend, 

 but no flaps. The shells are less pointed, and are found fossil in the 

 newer tertiaries. A group of very transverse shells, with the hinge 



