ORIGIN AND SIGNIFICANCE OF SPINES 



51 



spinulosus,^^ Actinocrinus lobatus,^^ A. pernodosus, Oreaster 

 occidentalism 0. gigas, Retaster crihrosus, etc. 



The concentric lamin;u of growth in the Brachiopoda are 

 frequently differentiated into spinules; as in Siphonotreta 

 unguicuhita,^^ Schizamhon typicalis,^^ Spirifer fimbriatus,^^ 

 S. pseudolineatus,^^ S. setigerus^^ Cliothgris Royssii,'^^ etc. 

 Other species show the differentiation of the radii into 

 spines; as Acanthothijris spinosa''* and A. Doderleini.^^ In 

 others the strong concentric laminae passing over radii are 

 often infolded into spines; as in Atrypa spinosa.^^ 



Among the Mollusca innumerable examples could be cited 

 showing clearly the differentiation of various ornamental 

 features into spines. Some of these 4o 



have already been discussed, Imt 

 may be referred to again in this 

 connection. Thus an illustration 

 of the passage of concentric laminae 

 into spines is shown in Avicxda 

 sterna^ and Anomia aculeata^^ (fig- 

 ures 26 and 28) and Slargaritiphora 

 fimhriata, etc. Many species of 

 Gastropoda show the same types of 

 differentiation. The differentiation 

 of radiating lines or ridges into 

 spines is equally common, and is well 

 shown in Spondylus (iigures 12, 14, 

 30), and in Lima squamosus (figure 40). In most of these 

 cases the rib represents the progression of a fold in the 

 edge of the mantle, while the spine is a process of a con- 

 centric lamina, and is usually more or less flat or tubular. 

 Occasionally the rib becomes obsolescent, and is represented 

 by a row of spines ; as in some specimens of the gastropod 

 Crucibulum spinosum. When the radiating and concentric 

 ornaments are distinctly continuous, a reticulate or cancellate 

 appearance is produced, and the points of intersection often 

 bear spines; as in Avioulopecten scabridus,^^ A. ornatus^^ 

 Actinopteria Boydi^^ Pterinopecten spondylus,'^^ etc. 



Figure 40. — Lima squa- 

 mosus. Natural size. To show 

 differentiation of radii into 

 spines. 



