SILURIAN AC K 5 



This family includes tin- following genera represented in our existing Roas, \\/ . 

 ( 'iifilinn, /'I, in;, j, us, (_'li<>, /l<i/<mtiti>/i, ,SV///Wa ami Ctn-'urin. The extinct g'licra 

 nio-t probably belonging here, are '/'/// i/. Suit, />//<. \'<ii/in> /In and l't< r<>t]n<-a f The 

 fn-sil shells ( 'nun/, ifiu and ('i>/,/>n'n, are also generally referred to this family, but 

 if there is no mistake about the c \i-ti are of .epta and a sipliunrlc in the former 

 genus (sec Cnniiliirin tn-nhmeitHi*, Pal. X. Y. Vol. 1, p. '.''.'I, f. 4), it would be ex- 

 cluded, not only from this family, but probably from the subclass J'Urj>ila. 



Genus THECA, SOWERBT. 



Synon Thtca, J. de C. SOWIBBT, MSB., In Morris' Memoir, Strexelsckl's N. 8. Wales, 1845, p. 389. 



]*u 3 i H nculu>, BABBAHDE, Nenes Jahrb. far Min. 1847, p. 364. 

 Etymtit. 3'xt, a sheath, case or covering. 

 Typr.THeca lanceolata, MORRIS. 



Shell thin, more or less elongate-conical, nearly or quite straight ; aperture and 

 section trigonal or subtrigonal ; surface smooth or striated. (Animal unknown.) 



According to Prof. Barrandc (who had not seen the description of the genus 

 Theca at the time he proposed the name Puginncvlus), the aperture of these little 

 shells was closed by a triangular shelly operculum. This would seem to be a rather 

 anomalous character in the family to which this group appears to belong. 



The genus Theca commenced its existence amongst the earliest forms of life 

 during the primordial period, and its remains are found in various parts of the 

 Silurian system, both upper and lower; though it probably attained its greatest 

 development during the deposition of the Lower Silurian rocks. It is also known 

 to range up into the Devonian, two species having been described by Sandberger 

 from deposits of that age in the Rhcinish Provinces. As one species T. aculeata, 

 Hall has been discovered in beds in this country holding a position near the 

 dividing. line between Devonian and Carboniferous rocks, it is probable the genus 

 did not become entirely extinct until some time after the close of the Carboniferous 

 epoch. 



Theca 



Thtra (Puyiunculut) gregaria, MURK & HAYDBH, Proceed. Acad. Nat. Sol. Fhila., December, 1861, p. 436. DASA'S 

 Geology, p. 187, Fig. 244 B. 



Shell small, straight, acutely conical ; dorsal side compressed or nearly flat ; ventral side rounded ; lateral mar- 

 gins obtusely angular, and converging regularly at an angle of about 18 to the pointed lower extremity ; aperture 

 and transverse section nearly semicircular, or forming rather more than half a circle ; lip on the flat or dorsal side 

 somewhat produced, and rounded in outline ; surface of casts nearly or quite smooth. 



Length, 0.45 inch ; breadth, 0.16 inch ; convexity, 0.10 Inch at the aperture. 



d 



Tkeca gregaria. 

 a. Ventral tide. 6. Dorsal side. <. Side view. d. Transverse section. 



These delicate little- shells must have existed iu great numbers, since on a single 

 flat piece of sandstone, not more than sL\ by eight inches across, we have counted 



