THE CLISTENTERATA. 267 



buried in sand shoals or mud banks, and are occasionally left dry by the receding tide. They are 

 very abundant in certain localities, in from 7 to 60 fathoms in the tropics, the China and Japan seas, 

 and off the Australian and Pacific coasts. They are also plentiful at low water in the Philippine 

 Islands. Glottidia abounds on both coasts of North America. No member of this family inhabits deep 

 water. The " Helmet Shells " (Craniadse) exist in from 3 to 800 fathoms off the shores of Britain, 

 Japan, and Australia. The " Disc Shells " (Discinidse) are generally dredged in from 3 to 50 fathoms 

 oft' Japan and the South Pacific coasts. They range from Baffin's Bay to south of the Cape de 

 Verde Islands ; from the Arctic regions to the equatorial Atlantic. One interesting little species 

 of this family has succeeded in adapting itself to life in the abysses of the ocean, and enjoys a bathy- 

 metrical range of from 6 ( JO to 2,425 fathoms. It was dredged at this depth at several stations by 

 H.M.S. Challenge); from its home at the bottom of the Atlantic, whence its specific name of 

 Atlantica is derived. This species, however, also occurs off Australia, for, like all abyssal forms, it 

 enjoys a wide geographical distribution. It is the only member of the more highly organised group 

 (Tretenterata) which can really be considered an inhabitant of the deep seas. 



The species of the order Clistenterata are more frequently found in deep waters, althotigh 

 some, like the Australian Waldheimia, are merely washed by the tide and may be gathered by 

 the hand, "like limpets on the shore." A species of the genus Kraussina is also left dry 

 by the tide on that desolate rock of St. Paul's Island in mid-Atlantic. The family of 

 " little beaks " is represented in the New Zealand area, off Japan, in the Arctic and North and 

 South Atlantic Oceans, at depths varying from 10 to 690 fathoms. Thecidium (Fig. 13) inhabits 

 the Mediterranean Sea, and may also be sought off Jamaica and the Mauritius in from 30 to 

 300 fathoms. But it is among the universally- distributed Terebratulidse that the greatest variety 

 of depths has been recorded: from 200 to 600 fathoms being the usual limit of the majority 

 of species. Five, however, are known to live at from 1,000 to 1,500 fathoms; four in from 

 1,500 to 2,000, and three in from 2,000 to 2,900 fathoms. The Challenger dredged a 

 very pretty little species of Terebratula at the last-mentioned depth (Fig. 14). Its occurrence 

 off Valparaiso in 2,160 fathoms, and South Australia in 2,600 fathoms, affords another illustration 

 of the fact that the deep-sea Brachiopoda are uniformly and widely diffused. The same species 

 always recur at stations far distant from each other, a fact that is probably owing to the 

 uniformity of the temperature, which below a cei-tain bathymetrical limit never exceeds 

 a few degrees above freezing-point. It is also very remarkable that the shells of 

 the deep-sea forms are invariably delicate, and so transparent that the muscular im- 

 pressions and the shape of the loop can often be distinguished from the exterior. 

 Yet, notwithstanding the exceeding delicacy and fragility of their shells, the animals 

 sustained life at a depth where the pressure of the water exceeded two tons and Fig- 14. TE- 



... . r KEBHATULA 



a halt to every square inch or surface. WYVILLII. 



Thus it is evident that these shell-fish can adapt themselves to life under the 2,900 fa- 



most varying conditions of temperature and depth, whether in Arctic or Tropical ' 



regions, at low-water mark, or in the untroubled abysses of the ocean. Nor is their 



range in time less extended than their bathymetrical limits or present geographical distribution, 



for the Brachiopoda were among the first representatives of life in the primeval oceans. 



Truly of most ancient lineage, they are found in' the lower Cambrian strata, and formed a 

 very important feature of tho animal community in the following " ancient life " epoch, often called 

 the " reign of molluscs " from the numerical preponderance of those organisms. Members of 

 the Tretenterate group were, accor-ding to our present knowledge, the first to make their 

 appearance in the Cambrian seas, where the remains of the earliest known species, a Linguloid 

 form (Lingulella, primeva}, were embedded. Only two genera, Lingula and Discina (Figs. 7 

 and 2,), out of about 130, have persisted throughout all the geological ages up to the 

 present day. Crania, Rhynchonella, Terebratula, Waldheimia,* and Glottidia, a sub-genus of 

 Lingula, date from the Silurian. But the majority of the remaining 124 generic forms enjoyed 

 a comparatively brief existence, appearing in one era to vanish at or before its close. Thus, three 



* The genus, previously only known from the Jurassic upwards, was discovered by Mr. Davidson in strata of Upper 

 Silurian age. 



