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In some species , they are rather soft and transparent ; in others they have a hard , opaque , calcareous shell . This page will offer images , keys , and a variety of collection , literature and distribution information to facilitate our understanding of freshwater diatom biodiversity , systematics , and evolution . Line up 20 skulls of the same sex and species , and the differences between them pop out . His research interests include the behavioral ecology of vertebrates , conservation biology of declining species , and the application of innovative field techniques . This list is preliminary ; we anticipate the addition of many new species , both species new to science and species new for Myanmar . This family contains approximately 64 percent of the more than 2,500 snake species and includes such familiar and harmless ones as garter snakes , gopher snakes , racers , and rat snakes . The number of eggs in a clutch varies greatly , often 20-50 in large species , but sometimes 100 or more . For example , the many manifestations of inattention ( poor memory , concentration difficulties , disorganized behavior , distractibility , etc. ) are ubiquitous in psychiatry , and are observed in child , adolescent , and adult disorders . Venom toxicities are generally measured in terms of mouse LD-50s , which is the dose required to kill one-half of a test group of mice which have been given the venom . The childhood syndrome primarily consists of three core symptoms , inattention ( concentration difficulties , short attention span , distractibility ) , impulsivity ( impatience , acting without thinking ) , and hyperactivity ( restlessness , always on the go ) .