REPTILIAN AGE. JURASSIC PERIOD. 79 



Family PTERIIDtE. (See page 27.) 



Subfamily PTERIUST^. (See page 28.) 



Genus PTERTA, Scopoli. 



Si/Kon. — Pteria, Scopoli, Introd. Hist. Nat. 1777. — Gkat, ZooL Proceed. 1847,199. — Meek, Am. Jour. Sci. and Arts [2], 

 XXXVII, 1864, 217. 



Avicula (Klein), Beug (part), Enoyo. Meth. 1792, pi. 177. — Cuvier (part). Tab. Elem. 1798 ; Auat. Comp. 

 1800; Regne An. 1817.— Lamk. Prodr. 1799; Syst. An. 1801, 134; Phil. Zool. 1809, 318, &o. 



Margaritifera (sp.). Humph. Mus. Col. 1797, 44. 



Pinctada, Link, Beschr. Host. 1807 (not Bolten, 1798). 



Z/nionium, Link, ib. 



Anonica, Oken, Handb. d. Zool. 1815 ; Natgsoh. f. Schulen. 1815, 652. 



Perlamater (sp.), Schdm. Ess. 1817, 107. 

 Etym. — WTEfo'v, a wing. 

 Examp. — Mytilus hirundo, Linn. 



Shell obliquely subovate, or subtrigonal, fragile ; surface smooth, striated, costated 

 or subspinous — often with imbricating marks of growth ; inequality of valves gene- 

 rally distinctly marked ; byssal sinus in the anterior margin of the right valve well 

 defined. Cardinal margin long, straight, and produced into more or less distinct 

 wings at the extremities — the posterior wing being larger than the other. Hinge 

 with usually one or two small cardinal teeth under the beak of each valve. 

 The (simple) pallial line represented by a row of minute irregular scars, extending 

 from the subcentral impression of the adductor muscle obliquely forward to the 

 small anterior muscular scar beneath the beaks. 



Amongst the numerous fossil species referred by various authors to this genus, 

 there are, in addition to the several types we believe to belong to clearly distinct 

 genera, others which differ suiRciently from the living typical species to constitute 

 at least well marked sections. It is not our purpose, nor have we the necessary 

 material at hand, to attempt to define here ah of these various subordinate groups, 

 though it becomes necessary to notice one of those including a species with which we 

 have to deal. The section to which we allude may be designated as follows : — 



Oxytoma, Meek. 



Shell differing from the typical forms of Pteria (= Avicula), in being less oblique, proportionally shorter, more 

 distinctly inequivalve, and usually more strongly costate — particularly on the left valve, around the pallial 

 margins of which the costse are sometimes produced in the form of free spines. The byssal sinus of the 

 right valve is also much deeper and more sharply defined than in the typical species of Pteria. 



Type Avicula Munsteri, Goldf. Petref. Germ. II, pi. cxxviii, 2 a, to h.' Also includes A. costata, Moekis & 

 Lycett, a. digitaia, and apparently Monotis interlasvigata, Quenstedt, and A. cygnipes, Phillips. 



This section forms a transition from the typical Avicula to the genus Eumicrotis, and seems to be mainly, if not 

 entirely, confined to the Jurassic rocks. 



The genus Pteria, or Avicula, is so nearly related to the older extinct genus 

 Pterinia, that the two groups are generally confounded, where the hinge and in- 



* Several species appear to have been confounded under the name A. Munsteri, by other authors. 

 We regard the particular form figured by Gold^Bss as the type of the section Oxytoma. 



