BRITISH FOSSILS. 



5 



Fig. 3. Portion of the under surface of the fringe, magnified, showing the wide space 

 between the second and third rows of pores, and at a, the thick flattened edge. 

 The fringe is hollow, and its substance very thin. 



!Fig. 4. The posterior angle, magnified ; a raised edge, a, separates the fringe from the 

 spine ; the latter is often broken off at this point. 



rig. 5. Extremities of two thorax rings, with strong indentations ; a, falcral point. 



rig. 6. Head of fig. 2, magnified. At a, the indentations (for the attachment of the 

 hypostome ?) on each side is shown. 



Fig. 7. A section of the head and fringe, viewed rather from the upper side ; a, the 

 concavo-convex fringe ; b, the narrow raised ridge between the fringe and 

 the moderately convex glabella c ; at c? the cervical spine is shown. 

 All the specimens in the Mus. Practical Geology. 



The name of this genus can only be retained by general consent, for the typical species 

 was formerly denominated Cryptoliihus, and sufficiently described by Green ; and had, 

 indeed, received the name Nuttainia a few months earlier in the " Geological Text Book " 

 of Eaton, the American geologist. But in this case strict priority may be allowed to 

 yield to classical feeling, — the name Trinucleus, a strictly appropriate one, having been 

 used in one of the earliest figures given of these or any trilobites, viz., in Dr. Uhwyd's 

 paper in the Philosophical Transactions for August 1698. The ' Trinucleum fimhriatum^ 

 there figured, along with other trilobites, is the common Llandeilo species, now called 

 T. concentricus or T. Caractaci. 



In a short communication to the Geological Society, read March 1847, 1 endeavoured 

 to explain the structure of the peculiar fringe of this genus, which had been beautifully 

 figured just before by M. Rouault. However irregularly scattered the pores may seem in 

 some of the species, they can generally be traced as arranged in radiate lines ; in T. 

 radiatus and T. fimbriatus very strikingly so indeed. 



If these holes were elongated in the direction of the radii, so as to coalesce with each 

 other, the intervening ridges would become hollow spines standing out from the head 

 margin, and we should then at once recognize them as identical in structure with the 

 marginal spines so characteristic of Acidaspis, and a few other genera. On the other 

 hand, in the genus Harpes, not yet published in these Decades, the separation of the 

 expanded fringe has not proceeded so far as in Trinucleus, the puncta in that genus not 

 even piercing through the fringe, but only impressed upon it. 



This genus, like so many others, is now ascertained to undergo metamorphosis, at least 

 60 far as increase in the number of thorax rings is concerned, M. Barrande having found 

 the common species with from 0-6 body rings ; and a specimen of it with four rings 

 furnished M. Corda with materials for the foundation of his genus Tetrapsellium, a name 

 which must of course be cancelled. The late division of the genus by Professor M'Coy 

 into Trinucleus and Tretaspis, depends partly on this accidental circumstance ; but the 

 group Tretaspis will form a convenient sub-genus, distinguished by the other characters 

 he has pointed out, — the glabella furrows, the more distinct ocular tubercle, and facial 

 suture, &c. 



Trinucleus frequently occurs in a rolled-up form, as figured by Beyrich and Rouault. 

 The genus appears to us strictly Lower Silurian ; the specimens said to have been 

 obtained from Wenlock Shale are not yet well authenticated. 



Section I. Tkinucleus proper. 



1. T. Lloydii. Above described. 



2. T. concentricus, Eaton. 7>^nMC?eMm^m6r^a5^Mm z;M?(/are, Llhwyd (1698), Phil. Trans., 

 V. XX. tab, add. f. 9. Ichnogr. Brit. (1690), tab. 23, at top, Trilob. Brongniart, Crust 

 Eoss., t. 4. f. 6, 7. Bigsby, Ann. Lyc, Nat. Hist. New York, 1824, vol. i. pi. 15. f. 1. 

 Nuttainia concentrica, Eaton, Geol. Text Book (1832), pi, 1. f. 2. Hall, Pal. New York 

 (1847), pi. 65 and 67. T. Caractaci, Murch. Sil. Syst., pi. 23. f. 1. Ampyx (^Cryptol.) 



