LAMELLIBRANCHIATA. | UPPER PALAZOZOIC MOLLUSCA. 493 
2nd Order. IsrpDRoLoTILA. See page 265. 
2nd Family. MYTILIDA. See page 265. 
Genus. LITHODOMUS (Cur.) 
< Modiola Vam. 
Gen. Char.—longate, cylindrical (section circular) ; anterior end obtusely rounded; posterior end com- 
pressed; margins closed all round, no byssal ventral sinus ; one small anterior adductor, and one large superficial 
posterior one, connected by the entire pallial scar; no hinge-teeth; cartilage on a very small ridge nearly 
parallel throughout with the hinge-line ; beaks minute, inrolled, close to the anterior end. Animal: mantle open 
in the middle, closed anteriorly, and prolonged posteriorly into two very long, extensile tubes (not indenting the 
pallial scar) joined together, the anal one only open at the end, the other slit throughout, its edges continuous 
with those of the mantle ; gills forming long fringes of free filaments ; mouth with lips: foot very small, bilobed ; 
byssus on a separate tubercle in advance of it. 
Live burrowed in stone and corals, which they often line with a calcareous tube, having two openings at 
the narrow posterior outer end. Differs from Modiola in cylindrical shape, lithophagous habit, inrolled beaks, 
structure of the gills, closed anterior part of ‘the mantle, siphons, &e. 
Liruopomus JenKinsoni ((/‘Coy). PI. 3. F. fig. 2. 
Desc.—Longitudinally oblong, or oval, subeylindrical ; beaks small, much incurved, obliquely inrolled over 
the wide, deep, cordate, anterior lunette ; anterior end very short, extending very slightly in front of the beaks, 
obtusely rounded; posterior end slightly wider than the anterior, obtusely rounded; dorsal margin nearly 
straight, gradually rounding into the posterior end; ventral margin very slightly convex; valves evenly tumid, 
most so along an undefined line from the beaks to the respiratory margin a little in front of the middle of the 
length ; surface with irregular, coarse, concentric lines and plicee of growth. Casts shew the large anterior and 
posterior adductor impressions distinctly, connected by the simple pallial scar, together with faint traces of the 
concentric plicze of the surface, crossed by microscopic, close, diagonal strice from the beaks towards the respi- 
ratory margin, also impression of the small cartilage ridge within the dorsal margin. Length one inch seven 
45 
lines, proportional greatest depth at middle of length 4, depth of posterior end ;, from apex of beaks to 
ventral margin *;, length and width of anterior lunette =, greatest depth of both valves =, length of anterior 
end =. 
This is a smaller and much more obtuse species than the LZ. dactyloides (M°Coy), the anterior end being 
proportionally broader and more obtusely rounded; the posterior end is also less pointed, and the anal angle 
not elevated. I have not seen the external shell of this species except near the margins, and there, there is no 
trace of the distinct longitudinal or radiating striation of that species, traces of which are however seen on the 
internal casts. 
I dedicate this species to the Rev. Mr Jenkinson, of Lowick, to whose labours we owe the most exten- 
sive and beautifully perfect local collection perhaps ever made in these rocks—particularly instructive by the 
frequent exhibition of internal characters. 
Position and Locality —Not uncommon in the impure carboniferous limestone of Lowick, Northumberland. 
Explanation of Figures.—P1. 8. F. fig. 2. Left valve natural size; exhibiting faintly the large adductor 
impressions, and entire pallial scar. 
