THE LAMELLIBRANCHIA 



259 



wards. This genus was very abundant in the Secondary epoch, particularly 



in Jurassic seas. There are six living species, all of which live in 



Australian seas. The animal of Trigonia was first found by Quoy and 



Gaimard in 1827. Schizodus, 



King ; from the Permian. Myo- 



phoria, Bronn ; from the Trias. 



FAMILY 7. LYRODESMIDAE, 



Ulrich. Shell inequilateral, the 



posterior side being the shorter. 



The hinge short, bearing teeth 



disposed in the shape of a fan. 



An extinct family, from the 



Silurian. Genus Lyrodesma, 



Conrad. 



SUB-ORDER 3. MYTILACEA. 



Symmetrical Lamellibranchia 

 in which the anterior adductor 



muscle is alvvavs less developed Philobrya sublaevis, Pels., viewed from the left 

 .lopea si(le . the left palljal lobe removed- ad t posterior 



than the posterior (the " aniso- adductor muscle ; an, anus ; au, auricle of the 



heart ; br, gill ; br.s, branchial axis ; by, byssus ; 

 gl.ge, gonad ; n.pa, pallial nerve ; p, foot ; pa, 

 mantle ; pal, labial palp ; re.p, re.p', anterior and 

 posterior retractor muscles of the foot ; st, 

 stomach ; ven, ventricle of the heart. 



FIG. 234. 



j-.- <. i , 

 myarian condition) or IS absent 



(Fi" 193 BCD E). The 



^ 

 heart gives oft a Single vessel 



only, the anterior aorta. The 



gills are smooth, the gill- filaments all alike and provided with inter- 

 lamellar junctions. The gonads generally extend into the mantle and 

 open at the sides of the kidneys. The foot is linguiform and byssiferous. 

 FAMILY 1. MYTILIDAE, d'Orbigny. Shell inequilateral, the anterior 

 side being short ; the hinge without teeth ; the ligament external. The 

 mantle has a posterior suture. Cephalic eyes present. Genera Mytilus, 

 Linnaeus ; the shell with terminal umbones ; British. Modiola, Lamarck ; 

 the umbones behind the anterior extremity ; British. Lithodomiis, Cuvier ; 

 the shell sub-cylindrical, adapted to boring. Modiolaria, Loven ; posterior 

 pallial orifice provided with an elongated siphon ; anterior adductor 

 fairly high ; British. Crenella, Brown. Stavelia, Gray. Dacrydium, 

 Torell. Myrina, Adams. Idas, Jeffreys. Septifer, Recluz. FAMILY 2. 

 MODIOLOPSIDAE, Fischer. Shell elongate, thin, inequilateral, enlarged 

 posteriorly ; the ligament external ; the adductor muscles subequal. An 

 extinct family from the Silurian to the Cretaceous. Genera Modiolopsis, 

 Hall ; from the Silurian. Modiomorpha, Hall ; from the Devonian. 

 Myoconcha, Sowerby ; from the Carboniferous to the Cretaceous. FAMILY 

 3. PERNIDAE, Fleming. Mantle open throughout. No anterior adductor 

 muscle. Shell very inequilateral ; the ligament multiple and lodged in a 

 series of vertical fossae. Genera Perna, Bruguiere. Shell sub-quad- 

 rangular, the right valve notched for the passage of the byssus ; gills free 

 posteriorly. Crenatula, Lamarck ; shell thin, flattened, irregular, without 

 a byssal notch ; inhabits sponges. UaJcewellia, King ; fossil from the 

 Permian. Gervilleia, Defrance ; fossil from the Trias to the Eocene. 



