6 G. LINDSTRÖM, A SILURIAN CYATHASPIS FROM GOTLAND. 
rhomboid shape with ridges divided into two fields, a narrow 
one at right angles to a larger on which the ridges descend, 
giving the whole a certain likeness to a comb. These ridges 
are of the same structure as the ridges of the secutum. The 
inferior margin is arched outwards, while the superior 18 
curved inwards. Besides this scale there is on the same slab 
an impression (P1. I, fig. 8) of a larger scale which seems to have 
been lying in its original place, behind the dorsal scutum 
Inv thes tant 
The dimensions of the different shields and their com- 
ponent parts are as follows. 
The dorsal shield. 
Hen ot 4 AL Ir FIN fan. EV VAT SINAN 
Breadth at äm Mar SM Ae ATA I. SIN 
> across the middle line -. . . - 30 mm. 
> at the posterior margin -. . . 20 mm. 
Dimensions. of £OStrmNy sk. FE RTR 65 
» of central plate: 34 42085 25mm 
Length of the most complete side lar. 34 mm. 
Breadth' of;Same. xc ch beer Fes Sa 
Thickness of the shell food 1 mm: at tihemöst. 
The total length of the ventral eld 44 mm. 
Greatest breadtblulsarkh 0 SEEN NE 
Breadthbat rostral mar gm Ich FSL 
"Dimensions of thezscale 3 kass on Al OR 
Length of the supposed limb . . . . . 25 mm. 
Greatest breadth 5. AA. st ALOE KG 
Thickness, nearly. . . SR One 
As to the histology 2 He Esks it may be said, that 
the different strata that can be discerned are by no means 
distinetly separated from each other, they are rather in 
uninterrupted continuation, or, as it were, modifications of 
one and the same stratum. The interior part consists of 
finely laminated strata of various shades of colour some 
darker, others lighter, and these pass imperceptibly upwards 
to form the partition walls between the large vacuoles, 
which constitute the second stratum or the main of the shell. 
The basal stratum (P1. II, fig. 1—4, a) is perforated by narrow 
tubes, some straight, others winding, which osculate both 
interiorly towards the cuticulum of the animal, and into 
the large vacuoles and thus have formed ducts between the 
msn Näsa SRA Arn GNRRER äraa nn han nad aln nd — Ål 
EES SSI 
