442 Lindstrom — On the Genus Trimerella, 



concentric layers. Some faint longitudinal strise are seen on the 

 interior walls of the siphons. The concentric layers around the 

 siphons form two strata that are quite distinct from the rest of the 

 shell-matter, and are imbedded in it. They cannot, therefore, bo 

 confounded with septa, which, when they do occur in the shells, are 

 in immediate contact with the valves, and compactly united with 

 them. The siphons of the dorsal valve are shorter than those in the 

 ventral valve, and often more divergent. I think we gain the true 

 interpretation of the nature of these siphons, if we attentively 

 examine the interior surface of the valves in the genera Lingula and 

 Oholus. The corresponding part of the valve of Lingula is occupied 

 by two impressions of the adductors, situated on each side of a 

 broad, faint, shield-like elevation. 



In some of the Silurian Lingulce these muscular impressions ^ are 

 somewhat excavated in the valve. 



The straight ridge, that continues below the impressions of the 

 adductors, and to which in the recent Lingulcd the " occlusores 

 anteriores,'^ Owen ("anterior retractors," Woodward), are fixed, is, 

 as it seems to me, homologous to the median ridge, continuing so 

 far below the openings of the siphons. This ridge consists (in 

 Trimerella) of two layers, that extend towards the margins of the 

 valves, and are an immediate continuation of the siphonal layers. 

 In Oholus the adductors occupy the same place as in Lingula, but are 

 separated by a larger elevation, or median shield. In some of the 

 Upper Silurian Oholi (which, perhaps, form a separate genus), this 

 median shield is very much elevated and prolonged in a long narrow 

 ridge, as in Lingula. In these Oholi the impressions of the adductors 

 are deeply hollowed in the valves, so that the casts resemble two 

 very short and pointed teeth.^ But this peculiar form of the muscular 

 scars is fully developed in Trimerella. I consider these characteristic 

 siphons to be the muscular scars, because their openings occupy the 

 same places as the adductors in the above-named genera, and also 

 because the walls are marked with those longitudinal striae so 

 characteristic of the muscular impressions ; they have, moreover, 

 a mode of development quite different from that of the septa. There 

 can, then, be discerned an almost continuous series in this develop- 

 ment of the muscular scars ; faintly indicated in the Lingulce, strongly 

 expressed in Oholus (more especially in those from the Upper 

 Silurian rocks), and fully developed in Trimerella. No other vestiges 



1 Davidson, Silurian Brachiopoda, pi. ii., fig. 35 ; pi. iii., figs. 5, 6. 



* Davidson (1. c. pi. iv., fig. 39). Compare also fig. 4 of my plate with tlie figure 

 of a dorsal valve of 'Trimerella grandis in the work of Mr. Billings. 



My above stated opinion as to the nature of the siphons of Trimerella is corro- 

 borated by the fact that in some species of Crania, especially the Cretaceous, these 

 scars of the adductors continue in the form of narrow pointed siphons, ceasing at the 

 apex of the valves. Thus, for instance, in Crania spimdosa, Hisinger, Leth. Suecica, 

 pi. xxiv., fig, 7«; also, C. Br attenhurgensis, in the German edition of Davidson's 

 "Introduction to the Brachiopoda,'' pi. v., fig. 15, and C. antiqua, Defr., pi. v., fig. \Qc. 

 I can also cite the valuable authority of the late Dr. S. P. Woodward, in his " Manual 

 of the Mollusca," p. 236 {Crania), "The large muscular impressions are , . . 

 sometimes deeply excavated." 



