RAMBLES OF A GEOLOGIST. 395 



most instances, when at its fiill size, had, like the coal-fish, 

 congregated in shoals when in a state of immaturity. But 

 a more careful examination of the specimens leads me to con- 

 clude that this minute gregarious Coccosteus, so abundant in 

 this locality that its fragments thickly speckle the strata for 

 hundreds of yards together (in one instance I found the 

 dorsal plates of four individuals crowded into a piece of flag 

 barely six inches square) was in reality a distinct species. 

 Though not more than one-fourth the size, measured linearly, 

 of the Goccosteus decipiens, its plates exhibit as many of those 

 lines of increment which gave to the occipital buckler of the 

 creature its tortoise-like appearance, and through which plates 

 of the buckler species were at first mistaken for those of a 

 Chelonian, as are exhibited by plates of the larger kinds, with 

 an area ten times as great ; its tubercles, too, some of them 

 of microscopic size, are as numerous ; evidences, I think, 

 when we take into account that in the bulkier species the 

 lines and tubercles increased in number with the growth of 

 the plates, and that, once formed, they seem never to have 

 been affected by the subsequent enlargement of the creature, 

 that this ichthyolite was not an immature, but really a 

 miniature Coccosteus. We may see on the plates of the 

 full-grown Coccosteus, as on the shells of bivalves, such as 

 Cardium echinatum, or on those of spiral univalves, such as 

 Buccinum undatum, the diminutive markings which they bore 

 when the creature was young ; and on the plates of this spe- 

 cies we may detect a regular gradation of tubercles from the 

 microscopic to the minute, as we may see on the plates of 

 the larger kinds a regular gradation from the minute to the 

 full-sized. The average length of the dwarf Coccosteus of 

 Thurso and Kirkwall, taken from the snout to the pointed 

 termination of the dorsal plate, ranges from one and a-half 

 to two inches ; its entire length from head to tail probably 

 from three to four. It was from one of Mr Dick's specimens 



