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 7rigonia was first found by Quoy and 
Gaimard in 1827. Schizodus, 
King ; from the Permian. Myo- 
phoria, Bronn ; from the Trias. 
Famity 7. LyrRoDESMIDAE, 
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. 
Sus-OrDER 3. MyTILAcea. 
Symmetrical Lamellibranchia 
in which the anterior adductor 
Philobrya sublaevis, Pels., viewed from the left 
muscle is always less developed side ; the left palliallobe removed. ad”, posterior 
than the posterior (the “aniso- adductor muscle; an, anus; aw, auricle of the 
o5 sy: : heart; br, gill; br.s, branchial axis; by, byssus; 
mpyerran condition) or is absent gl.ge, ;gonad ; .pa, pallial nerve; Pp, foot ; pa, 
Bye ee eg a LRG eens weantor abeee er tay tek ae 
heart gives off a single vessel stomach ; ven, ventricle of the heart. 
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. 
Faminy 1. Myrimipar, 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. Modtola, Lamarck ; 
the umbones behind the anterior extremity ; British. Lcthodomus, 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. Daerydiwm, 
Torell. Myrina, Adams. Idas, Jeffreys. Septifer, Recluz. Famrny 2. 
Moprotopsipak, 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. Faminy 
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, Bruguitre. 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. Bakewellia, King ; fossil from the 
Permian.  Gervilleia, Defrance ; fossil from the Trias to the Eocene. 
Fic. 234, 
