1848.] SHARPE ON THE GEOLOGY OF OPORTO. 151 



Tail convex, very broad, considerably wider than the head, and 

 projecting far beyond the sides of the head when the animal is rolled 

 up ; its anterior margin lobed, with a waved outline ; furrows marking 

 the axis very faint ; axis very small ; its length equal to about one- 

 third, and its breadth only equal to two-sevenths of that of the tail. 



Length of the animal about 3 inches. 



Head : length 1 inch ; greatest breadth 1-|- inch. 



Tail: length l^inch; greatest breadth l|inch. 



Found in slate at Vallongo. 



Unfortunately the head of the specimen has been crushed in at the 

 sides, which makes it doubtful how far the triangular outline it now 

 presents is natural ; and the general condition of the specimen does 

 not admit of a good description of the species. 



The most marked peculiarity seen is the great breadth of the tail, 

 both compared to the head and to its own axis, which occupies little 

 more than one-fourth of the breadth of the tail. 



The I. giganteus of the Angers slates, Burm. t. 3. f. 10, is the 

 only described species to which this can possibly be related, having 

 the same narrow caudal axis ; but the Portuguese trilobite has a far 

 broader tail than is shown in the only figure yet published of that 

 species. Unfortunately the /. giganteus is so little known and so 

 slightly figured, that we have not the means of any certain comparison 

 with it. 



Fig. 1 a. PI. VI. head and part of the body. 



Fig. 1 b. PL VI. tail of the same specimen. 



Orthis Noctilio, n. sp. 



Shell nearly flat ; outline semi-ovate with sides nearly straight ; 

 length about two-thirds of the width ; thickness inconsiderable. 



Dorsal valve slightly convex, with an elevated mesial ridge. 



Ventral valve nearly flat, with a mesial furrow corresponding to the 

 ridge of the dorsal valve. 



Hinge-areas narrow, slightly produced beyond the sides of the 

 shell. 



Surface covered with fine concentric lines, and ornamented with 

 narrow unequal ribs which bifurcate several times, being about 1 5 in 

 number near the hinge, 50 at the middle of the valve, and between 

 60 and 70 at the margin. 



Interstices wider than the ribs. 



Interior margins of the valves strongly crenated. 



Length 1^ inch ; width at the hinge 2\ inches. 



Found in slate near Vallongo. 



The elevated ridge on the dorsal valve connects this species with 

 the O. hilohata^ Sow., and O. vespertilio, Sow., which are found in the 

 Lower Silurian rocks of this country : it is distinguished from both 

 those shells by coarser, simpler and less numerous ribs. 



Fig. 2 a. PI. VI. Ventral valve, from the cast of adistorted specimen. 



Fig. 2 b. Ventral valve ; interior of the same specimen. 



Fig. 2 c. Interior of dorsal valve. 



