98 



Guide to the Invertehrata. 



case 17. 



Ophiu- In the Jurassic period there are some well-preserved brittle-stars 



roidea. referred to the genera Ophioderma, Ophiolepis, and Ophiurella. 

 ^^ni^"^ The collection includes many of the types described by Wright, 

 Table-case ^^^^ ^^^o the specimens of Ophioglypha serrata, from the Chalk, 

 75, Wall- figured by Dixon. Ophioglypha Wetherelli, from the London Clay, 

 is the most important of the British Cainozoic brittle-stars, while 

 the Pleistocene deposits of Western Scotland have yielded a species, 

 Ophiolepis gracilis, AUm., which is well represented by three 

 specimens (40,220). 



The foreign Ophiuroids are on the lowest shelf of Wall-case 17a. 

 In the Mesozoic series the four best forms are Geocoma Lihanotica 

 from the Cretaceous limestones of the Lebanon, Geocoma carinata 

 and Ophiurella speciosa from the Solenhofen Slates, and a small 

 Hemiglypha from the Muschelkalk. 



The Palaeozoic fauna is represented by species of Protaster and 

 Onycha^ter flexilis, M. and W., from the Carboniferous rocks of 

 Indiana, the latter of which has coiled arms like the Eucladia. 

 The most valuable series is, however, that from the Devonian Slates 

 of Bundenbach, in Germany, including Stiirtz's type-specimens, 

 which arc shown in a table-case in the middle of the gallery. 

 The species of Ophiurina and Eoluidia are the most instructive. 



Y. — Ckinoidea. 



Crinoidea. This class is represented in modern seas, at all depths, by six 



Table- genera with stalks (e.g. Pentacrinus, Rhizocrinus) and six without 



73'&'74 ^^'^' ^^^^^^^) ' the forms named are exhibited for comparison 



Wall cases with the fossils. Whether stalked or unstalked, the crinoid 



^^~^°- has a calyx of varying size, in which its viscera are packed, 



and five flexible arms, which are often much branched and 



often provided with small side-branches or pinnules. The stalk 



and fern-like arms give these animals a very plant- like aspect; 



hence the name crinoid, meaning "lily-form." The mouth is 



placed on the top of the calyx, between the arms. Each arm 



is grooved on its inner surface, and water containing the animalculae 



on which the crinoid feeds is swept down the grooves to the mouth. 



The nutritive stream passes through the gut coiled inside the 



calyx, and out again at the anus, which is often at the end 



of a long tube (e.g. Actinocrinus). Further details as to the 



structure of a crinoid are explained in Table-case 73. The 



