714 LAMELLIBRANCHIATA. 



Limoftsis, ranging from the Trias to the present day, has an orbicular, 

 but slightly oblique shell (fig. 583, d), with a central triangular cartilage- 

 pit, and a row of transverse teeth on each side of this. Lastly, the Mio- 

 cene and Pliocene genus Nuculina has the shell oval or subtrigonal, 

 with the anterior side abbreviated ; the hinge-line being curved, and 

 carrying a row of short transverse teeth completed posteriorly by one 

 elongated lamelliform lateral tooth. 



In connection with the present family we may consider the ex- 

 tinct genera Cardiola and Cyrtodonta, with some forms allied to the 

 last of these, the precise affinities of these being 

 uncertain. In the genus Cardiola (fig. 584) 

 the shell is thin, equivalve, and oblique, with 

 convex valves and prominent incurved beaks, 

 and having the surface adorned with radiating 

 grooves, which are crossed by concentric fur- 

 rows in such a way as to give rise to radiating 

 rows of tubercular eminences. There is a large 

 triangular, transversely-grooved ligamental area 

 below the beak, and the hinge-line is straight ; 

 but teeth have not been certainly detected in 

 Fig. 584.— Cardiola cor- the typical forms of the genus, and the mus- 



nucopiee, from the Devon- , . . , x T 



ian rocks of Germany, of cular impressions are unknown. Numerous 

 attei.) tural S1Ze " ter species of the genus have been recognised in 

 the Silurian and Devonian rocks, a character- 

 istic species from the former system being the well-known Cardiola 

 interrupta. 



The shells of the genus Cyrtodonta {Palcearca of Hall) are ventri- 

 cose and very inequilateral, the umbones being anterior (fig. 585). 

 The hinge-area is undefined, and the surface is generally smooth. 

 There are a few (three) anterior cardinal teeth, and " two or three 

 remote oblique posterior teeth parallel to the hinge-margin " (Salter). 

 The species of Cyrtodonta appear to be exclusively confined to the 

 Ordovician, Silurian, and Devonian rocks, but the genus is said to 

 occur in the Upper Cambrian. Vanuxemia comprises forms of the 

 genus with nearly terminal beaks. In the genus Mega/omus, again 

 (fig. 586), are comprised forms essentially similar to Cyrtodonta 

 (possibly identical with it), but having an excessively thickened and 

 massive shell, which is usually strongly inflated. The valves are 

 equal ; the beaks strongly incurved and placed anteriorly ; the hinge- 

 line furnished with three or four (?) strong transverse teeth ; and the 

 anterior muscular impression extremely deep. The forms of this 

 genus appear to be confined to the Silurian rocks, and often attain a 

 great size, but they usually occur in the state of casts only. 



Family 2. Nuculid^e. — This family comprises a number of marine 

 Bivalves, in which the mantle-lobes may be open, or siphons may be 



